Why do I have to disable a proxy in apt.conf.d?

Question

I asked the following question when apt-get update was failing to resolve hosts:

Could not resolve archive.ubuntu.com

Turns out there was a proxy set up for apt-get, and by adding a file to /etc/apt/apt.conf.d/ with the following contents fixed it:

Acquire::http::Proxy "false";

But now I'd like to find out why. Clearly something is setting up a proxy to begin with. How can I track this down? I have tried running grep -ri proxy /etc/apt/apt.conf.d/*, which turned up nothing.

EDIT: as suggested, here is the output of sudo apt-config dump:

APT "";
APT::Architecture "amd64";
APT::Build-Essential "";
APT::Build-Essential:: "build-essential";
APT::Install-Recommends "1";
APT::Install-Suggests "0";
APT::Sandbox "";
APT::Sandbox::User "_apt";
APT::Authentication "";
APT::Authentication::TrustCDROM "true";
APT::NeverAutoRemove "";
APT::NeverAutoRemove:: "^firmware-linux.*";
APT::NeverAutoRemove:: "^linux-firmware$";
APT::NeverAutoRemove:: "^linux-image-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^linux-image-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^linux-image-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^linux-headers-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^linux-headers-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^linux-headers-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^linux-image-extra-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^linux-image-extra-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^linux-image-extra-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^linux-signed-image-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^linux-signed-image-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^linux-signed-image-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^kfreebsd-image-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^kfreebsd-image-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^kfreebsd-image-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^kfreebsd-headers-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^kfreebsd-headers-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^kfreebsd-headers-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^gnumach-image-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^gnumach-image-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^gnumach-image-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^.*-modules-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^.*-modules-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^.*-modules-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^.*-kernel-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^.*-kernel-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^.*-kernel-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^linux-backports-modules-.*-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^linux-backports-modules-.*-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^linux-backports-modules-.*-4\.4\.0-45-generic$";
APT::NeverAutoRemove:: "^linux-tools-4\.4\.0-34-generic$";
APT::NeverAutoRemove:: "^linux-tools-4\.4\.0-36-generic$";
APT::NeverAutoRemove:: "^linux-tools-4\.4\.0-45-generic$";
APT::VersionedKernelPackages "";
APT::VersionedKernelPackages:: "linux-image";
APT::VersionedKernelPackages:: "linux-headers";
APT::VersionedKernelPackages:: "linux-image-extra";
APT::VersionedKernelPackages:: "linux-signed-image";
APT::VersionedKernelPackages:: "kfreebsd-image";
APT::VersionedKernelPackages:: "kfreebsd-headers";
APT::VersionedKernelPackages:: "gnumach-image";
APT::VersionedKernelPackages:: ".*-modules";
APT::VersionedKernelPackages:: ".*-kernel";
APT::VersionedKernelPackages:: "linux-backports-modules-.*";
APT::VersionedKernelPackages:: "linux-tools";
APT::Never-MarkAuto-Sections "";
APT::Never-MarkAuto-Sections:: "metapackages";
APT::Never-MarkAuto-Sections:: "contrib/metapackages";
APT::Never-MarkAuto-Sections:: "non-free/metapackages";
APT::Never-MarkAuto-Sections:: "restricted/metapackages";
APT::Never-MarkAuto-Sections:: "universe/metapackages";
APT::Never-MarkAuto-Sections:: "multiverse/metapackages";
APT::Move-Autobit-Sections "";
APT::Move-Autobit-Sections:: "oldlibs";
APT::Move-Autobit-Sections:: "contrib/oldlibs";
APT::Move-Autobit-Sections:: "non-free/oldlibs";
APT::Move-Autobit-Sections:: "restricted/oldlibs";
APT::Move-Autobit-Sections:: "universe/oldlibs";
APT::Move-Autobit-Sections:: "multiverse/oldlibs";
APT::Periodic "";
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Download-Upgradeable-Packages "0";
APT::Periodic::AutocleanInterval "0";
APT::Periodic::Unattended-Upgrade "1";
APT::Update "";
APT::Update::Post-Invoke-Success "";
APT::Update::Post-Invoke-Success:: "touch /var/lib/apt/periodic/update-success-stamp 2>/dev/null || true";
APT::Update::Post-Invoke-Success:: "[ ! -f /var/run/dbus/system_bus_socket ] || /usr/bin/dbus-send --system --dest=org.debian.apt --type=signal /org/debian/apt org.debian.apt.CacheChanged || true";
APT::Update::Post-Invoke-Success:: "if /usr/bin/test -w /var/cache/app-info -a -e /usr/bin/appstreamcli; then appstreamcli refresh > /dev/null; fi";
APT::Update::Post-Invoke-Success:: "/usr/lib/update-notifier/update-motd-updates-available 2>/dev/null || true";
APT::Archives "";
APT::Archives::MaxAge "30";
APT::Archives::MinAge "2";
APT::Archives::MaxSize "500";
APT::Architectures "";
APT::Architectures:: "amd64";
APT::Architectures:: "i386";
APT::Compressor "";
APT::Compressor::. "";
APT::Compressor::.::Name ".";
APT::Compressor::.::Extension "";
APT::Compressor::.::Binary "";
APT::Compressor::.::Cost "0";
APT::Compressor::lz4 "";
APT::Compressor::lz4::Name "lz4";
APT::Compressor::lz4::Extension ".lz4";
APT::Compressor::lz4::Binary "false";
APT::Compressor::lz4::Cost "50";
APT::Compressor::gzip "";
APT::Compressor::gzip::Name "gzip";
APT::Compressor::gzip::Extension ".gz";
APT::Compressor::gzip::Binary "gzip";
APT::Compressor::gzip::Cost "100";
APT::Compressor::gzip::CompressArg "";
APT::Compressor::gzip::CompressArg:: "-6n";
APT::Compressor::gzip::UncompressArg "";
APT::Compressor::gzip::UncompressArg:: "-d";
APT::Compressor::xz "";
APT::Compressor::xz::Name "xz";
APT::Compressor::xz::Extension ".xz";
APT::Compressor::xz::Binary "xz";
APT::Compressor::xz::Cost "200";
APT::Compressor::xz::CompressArg "";
APT::Compressor::xz::CompressArg:: "-6";
APT::Compressor::xz::UncompressArg "";
APT::Compressor::xz::UncompressArg:: "-d";
APT::Compressor::bzip2 "";
APT::Compressor::bzip2::Name "bzip2";
APT::Compressor::bzip2::Extension ".bz2";
APT::Compressor::bzip2::Binary "bzip2";
APT::Compressor::bzip2::Cost "300";
APT::Compressor::bzip2::CompressArg "";
APT::Compressor::bzip2::CompressArg:: "-6";
APT::Compressor::bzip2::UncompressArg "";
APT::Compressor::bzip2::UncompressArg:: "-d";
APT::Compressor::lzma "";
APT::Compressor::lzma::Name "lzma";
APT::Compressor::lzma::Extension ".lzma";
APT::Compressor::lzma::Binary "xz";
APT::Compressor::lzma::Cost "400";
APT::Compressor::lzma::CompressArg "";
APT::Compressor::lzma::CompressArg:: "--format=lzma";
APT::Compressor::lzma::CompressArg:: "-6";
APT::Compressor::lzma::UncompressArg "";
APT::Compressor::lzma::UncompressArg:: "--format=lzma";
APT::Compressor::lzma::UncompressArg:: "-d";
Dir "/";
Dir::State "var/lib/apt/";
Dir::State::lists "lists/";
Dir::State::cdroms "cdroms.list";
Dir::State::mirrors "mirrors/";
Dir::State::extended_states "extended_states";
Dir::State::status "/var/lib/dpkg/status";
Dir::Cache "var/cache/apt/";
Dir::Cache::archives "archives/";
Dir::Cache::srcpkgcache "srcpkgcache.bin";
Dir::Cache::pkgcache "pkgcache.bin";
Dir::Etc "etc/apt/";
Dir::Etc::sourcelist "sources.list";
Dir::Etc::sourceparts "sources.list.d";
Dir::Etc::main "apt.conf";
Dir::Etc::netrc "auth.conf";
Dir::Etc::parts "apt.conf.d";
Dir::Etc::preferences "preferences";
Dir::Etc::preferencesparts "preferences.d";
Dir::Etc::trusted "trusted.gpg";
Dir::Etc::trustedparts "trusted.gpg.d";
Dir::Bin "";
Dir::Bin::methods "/usr/lib/apt/methods";
Dir::Bin::solvers "";
Dir::Bin::solvers:: "/usr/lib/apt/solvers";
Dir::Bin::dpkg "/usr/bin/dpkg";
Dir::Bin::bzip2 "/bin/bzip2";
Dir::Bin::xz "/usr/bin/xz";
Dir::Bin::lz4 "/usr/bin/lz4";
Dir::Bin::lzma "/usr/bin/xz";
Dir::Media "";
Dir::Media::MountPath "/media/apt";
Dir::Log "var/log/apt";
Dir::Log::Terminal "term.log";
Dir::Log::History "history.log";
Dir::Ignore-Files-Silently "";
Dir::Ignore-Files-Silently:: "~$";
Dir::Ignore-Files-Silently:: "\.disabled$";
Dir::Ignore-Files-Silently:: "\.bak$";
Dir::Ignore-Files-Silently:: "\.dpkg-[a-z]+$";
Dir::Ignore-Files-Silently:: "\.save$";
Dir::Ignore-Files-Silently:: "\.orig$";
Dir::Ignore-Files-Silently:: "\.distUpgrade$";
Acquire "";
Acquire::AllowInsecureRepositories "1";
Acquire::AllowDowngradeToInsecureRepositories "0";
Acquire::cdrom "";
Acquire::cdrom::mount "/media/cdrom/";
Acquire::IndexTargets "";
Acquire::IndexTargets::deb "";
Acquire::IndexTargets::deb::Packages "";
Acquire::IndexTargets::deb::Packages::MetaKey "$(COMPONENT)/binary-$(ARCHITECTURE)/Packages";
Acquire::IndexTargets::deb::Packages::flatMetaKey "Packages";
Acquire::IndexTargets::deb::Packages::ShortDescription "Packages";
Acquire::IndexTargets::deb::Packages::Description "$(RELEASE)/$(COMPONENT) $(ARCHITECTURE) Packages";
Acquire::IndexTargets::deb::Packages::flatDescription "$(RELEASE) Packages";
Acquire::IndexTargets::deb::Packages::Optional "0";
Acquire::IndexTargets::deb::Translations "";
Acquire::IndexTargets::deb::Translations::MetaKey "$(COMPONENT)/i18n/Translation-$(LANGUAGE)";
Acquire::IndexTargets::deb::Translations::flatMetaKey "$(LANGUAGE)";
Acquire::IndexTargets::deb::Translations::ShortDescription "Translation-$(LANGUAGE)";
Acquire::IndexTargets::deb::Translations::Description "$(RELEASE)/$(COMPONENT) Translation-$(LANGUAGE)";
Acquire::IndexTargets::deb::Translations::flatDescription "$(RELEASE) Translation-$(LANGUAGE)";
Acquire::IndexTargets::deb::DEP-11 "";
Acquire::IndexTargets::deb::DEP-11::MetaKey "$(COMPONENT)/dep11/Components-$(NATIVE_ARCHITECTURE).yml";
Acquire::IndexTargets::deb::DEP-11::ShortDescription "Components-$(NATIVE_ARCHITECTURE)";
Acquire::IndexTargets::deb::DEP-11::Description "$(RELEASE)/$(COMPONENT) $(NATIVE_ARCHITECTURE) DEP-11 Metadata";
Acquire::IndexTargets::deb::DEP-11::KeepCompressed "true";
Acquire::IndexTargets::deb::DEP-11::KeepCompressedAs "gz";
Acquire::IndexTargets::deb::DEP-11-icons "";
Acquire::IndexTargets::deb::DEP-11-icons::MetaKey "$(COMPONENT)/dep11/icons-64x64.tar";
Acquire::IndexTargets::deb::DEP-11-icons::ShortDescription "icons-64x64";
Acquire::IndexTargets::deb::DEP-11-icons::Description "$(RELEASE)/$(COMPONENT) DEP-11 64x64 Icons";
Acquire::IndexTargets::deb::DEP-11-icons::KeepCompressed "true";
Acquire::IndexTargets::deb::DEP-11-icons::KeepCompressedAs "gz";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi "";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi::MetaKey "$(COMPONENT)/dep11/icons-128x128.tar";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi::ShortDescription "icons-128x128";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi::Description "$(RELEASE)/$(COMPONENT) DEP-11 128x128 Icons";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi::KeepCompressed "true";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi::KeepCompressedAs "gz";
Acquire::IndexTargets::deb::DEP-11-icons-hidpi::DefaultEnabled "false";
Acquire::IndexTargets::deb-src "";
Acquire::IndexTargets::deb-src::Sources "";
Acquire::IndexTargets::deb-src::Sources::MetaKey "$(COMPONENT)/source/Sources";
Acquire::IndexTargets::deb-src::Sources::flatMetaKey "Sources";
Acquire::IndexTargets::deb-src::Sources::ShortDescription "Sources";
Acquire::IndexTargets::deb-src::Sources::Description "$(RELEASE)/$(COMPONENT) Sources";
Acquire::IndexTargets::deb-src::Sources::flatDescription "$(RELEASE) Sources";
Acquire::IndexTargets::deb-src::Sources::Optional "0";
Acquire::Changelogs "";
Acquire::Changelogs::URI "";
Acquire::Changelogs::URI::Origin "";
Acquire::Changelogs::URI::Origin::Debian "http://metadata.ftp-master.debian.org/changelogs/@CHANGEPATH@_changelog";
Acquire::Changelogs::URI::Origin::Tanglu "http://metadata.tanglu.org/changelogs/@CHANGEPATH@_changelog";
Acquire::Changelogs::URI::Origin::Ubuntu "http://changelogs.ubuntu.com/changelogs/pool/@CHANGEPATH@/changelog";
Acquire::Changelogs::URI::Origin::Ultimedia "http://packages.ultimediaos.com/changelogs/pool/@CHANGEPATH@/changelog.txt";
Acquire::Changelogs::AlwaysOnline "true";
Acquire::Changelogs::AlwaysOnline::Origin "";
Acquire::Changelogs::AlwaysOnline::Origin::Ubuntu "1";
Acquire::http "";
Acquire::http::Proxy "false";
Acquire::Languages "";
Acquire::Languages:: "en_AU";
Acquire::Languages:: "en";
Acquire::Languages:: "none";
Aptitude "";
Aptitude::Get-Root-Command "sudo:/usr/bin/sudo";
Unattended-Upgrade "";
Unattended-Upgrade::Allowed-Origins "";
Unattended-Upgrade::Allowed-Origins:: "${distro_id}:${distro_codename}-security";
DPkg "";
DPkg::Pre-Install-Pkgs "";
DPkg::Pre-Install-Pkgs:: "/usr/sbin/dpkg-preconfigure --apt || true";
DPkg::Post-Invoke "";
DPkg::Post-Invoke:: "if [ -d /var/lib/update-notifier ]; then touch /var/lib/update-notifier/dpkg-run-stamp; fi; /usr/lib/update-notifier/update-motd-updates-available 2>/dev/null || true";
Binary "apt-config";
Binary::apt "";
Binary::apt::APT "";
Binary::apt::APT::Color "1";
Binary::apt::APT::Cache "";
Binary::apt::APT::Cache::Show "";
Binary::apt::APT::Cache::Show::Version "2";
Binary::apt::APT::Cache::AllVersions "0";
Binary::apt::APT::Cache::ShowVirtuals "1";
Binary::apt::APT::Cache::Search "";
Binary::apt::APT::Cache::Search::Version "2";
Binary::apt::APT::Cache::ShowDependencyType "1";
Binary::apt::APT::Cache::ShowVersion "1";
Binary::apt::APT::Get "";
Binary::apt::APT::Get::Upgrade-Allow-New "1";
Binary::apt::APT::Cmd "";
Binary::apt::APT::Cmd::Show-Update-Stats "1";
Binary::apt::APT::Keep-Downloaded-Packages "0";
Binary::apt::DPkg "";
Binary::apt::DPkg::Progress-Fancy "1";
Binary::apt::Acquire "";
Binary::apt::Acquire::AllowInsecureRepositories "0";
CommandLine "";
CommandLine::AsString "apt-config dump";