Packages that leave obsolete conffiles after upgrade.
dpkg-maintscript-helper rm_conffile via dh_installdeb package.maintscript files is the recommended way to clean them up.
There may be false positives, e.g. if a conffile was converted to a maintainer script managed configuration file.
The commandline to find these logs is:
COMMAND='grep -E "OBSOLETE CONFFILE"'