Who is sending to notify-send?

Question

After reinstalling some drivers, I am receiving a lot of annoying empty messages to my notify-send every 30 sec - 5 min.

# dbus-monitor "interface='org.freedesktop.Notifications'"

outputs

method call time=1512991561.538291 sender=:1.924 -> destination=:1.16 serial=6 path=/org/freedesktop/Notifications; interface=org.freedesktop.Notifications; member=GetServerInformation
method call time=1512991561.539311 sender=:1.924 -> destination=:1.16 serial=7 path=/org/freedesktop/Notifications; interface=org.freedesktop.Notifications; member=Notify
   string "notify-send"
   uint32 0
   string ""
   string "Message"
   string ""
   array [
   ]
   array [
      dict entry(
         string "urgency"
         variant             byte 1
      )
   ]
  int32 -1

Each time I receive an empty message.

How can I figure out what process is sending them?

Linux user-pc 4.4.0-96-generic #119-Ubuntu SMP Tue Sep 12 14:59:54 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux