Previosly published question categories that have been upgraded from pre-1.9 can get "lost" by being promoted to the site context. They are no longer accessible to their original creators without some work by the admin. This has the potential of being very difficult in the general case as you can't tell who is using what question banks and in what courses.

This report would provide this information. It would also (optionally) move the categories back to course context if they did not need to be published/shared in the first place.