Open LMS has created a set of reports for the site administrator to help find those files. The File storage usage report and the Manual backups report are in the Open LMS heading in the admin settings menu. The File storage usage report shows where the site storage is being used; what kinds of files (course files vs backup files and logs) are making up the total consumed storage for the site. 

The manual backups report shows where all the manually created backups are located on the site. Which courses and which users have backups stored in them and how much space that takes up.

The courses are linked so that you can head from the report to the specific course backup file management and take care of any unneeded files.