What do you get with this query on the DB for Moodle: select contenthash,filename from `mdl_files` where (`component` like "%backup%" and `filename` like "%.mbz%")
I ended up getting all of the automatic backup references, that I imagine most (if not all) are non-existent files. In the off hours of today I'll do a backup and try removing some references to a hidden/unused course reference.
Hopefully I won't break everything.