Do you have automated backups set to go to an alternative directory ... a directory outside of moodledata/filedir/? or set to go to both an alternative area + the normal file system?
select filename,filesize,filearea from `mdl_files` where `component` like "backup"
or similar query ...
The db query above will show true backup files. Backups can be made by teacher as well.
Files that are in trashdir were not found to be linked anywhere in the Moodle. There are no references to them in the DB as being linked and through a task/cron job will be removed after a period of time (thought it was 4 days). Is your cron job running often enough?
However, if in a space crunch, the contents of /moodledata/trashdir/ can be removed manually. And, there could be backup files in /moodledata/temp/backup/ *IF* some courses are failing to complete autobackups for some reason.
Files in /moodledata/temp/backup/ can also be removed manually in a space crunch.
'spirit of sharing', Ken