Icon file paths
The KDE is following the Freedesktop icon theme specification https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html . Directory layout: https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#directory_layout . From the Askubuntu: Where are icons stored? .
KDE4/Plasma4
Right clicking the desktop icon show an option to look the desktop file.
Either Properties > preview (Folder desktop).

or Icon Settings > properties (Default desktop).

If the icon can be found from the default locations then only the icon name will be shown.

If the icon is not at the default places then the whole path will be shown.

Finding things
Ubuntu find https://help.ubuntu.com/community/find
locate - list files in databases that match a pattern, i.e. find
inside updatedb's list
Ubuntu finding files https://help.ubuntu.com/community/FindingFiles
Command:
locate name.png
should show the location of the icon name.png - here it does.