Is it possible to list every file on the system that does not belong to a package? or if they have been modified?
Might need to use something like: apt-get, apt-files, dpkg-query, etc
For context, imagine inheriting an old server that can't be formatted, and you want to check that every file is as it should be... I know this won't be the case when upgrading between versions of Debian, or after removing a package without "--purge", as it seems to leave allot of (typically config) files behind.
Likewise if that server was to be replaced with a new server, you want to make sure all of the configuration differences (compared to the base install) have been transferred (or ignored if no longer relevant - e.g. a rouge "Port" line being added to sshd_config).
It would also help identify if anything has been installed without the use of apt-get.