Sometimes you add a temporary APT source to /etc/apt/sources.list, install same packages and remove the repository again. Or you install some .deb file directly, which you copied by hand to your environment.

Here is some handy shell command, to find those packages, which are

• installed on your system
• but have no version in any of the currently configured APT repositories:

dpkg-query -W -f '${Package}\n' |
xargs apt-cache policy |
sed -ne '/^ /{H;$!b};x;/\*\*\* [^\n]*\n[^\n]*\/var\/lib\/dpkg\/status/p'

How does it work:

• We use dpkg-query to get a list of currently installed packages.
• We pipe that list to apt-cache, which prints the list of all known package versions.
• We use a sed script to
  • collect all belonging to one package in the hold buffer
  • print out that buffer if the current version (marked by ***) being known in /var/lib/dpkg/status only.