I've installed some things manually in the past and would like to weed out all related files. So, I need a way to automatically find all the files (in /usr, for example) that are not included in any of the packages currently installed on the Debian system. However, I would also need to filter out the files that are created during package installation (by dpkg post-install scripts and similar things).
You could try something like this:
This way you will get all files that are in /usr but not any package file. As a first shot this could help you.