I don't know if this works here but I have a strange problem with El Capitan.
If I import an mp4 into iTunes it won't let me delete the original even with all related apps closed (iTunes, QuickTime, etc). If I open trash and click "delete immediately" and watch the section About this Mac. It will move it from "Movies" to "Other".
So while it deletes it from my sight I don't actually get the space back. I can do this until the disk is full and unusable.
I'm not too familiar with the inner workings of OS X. Is there a method or place I can look to find and remove these files?