I have about 650 files in my offline folder and I did not know how to delete them until just a few minutes ago. Someone posted and led me to understand that offline files are marked as “available offline” (not in my case) and that they are located in Google Drive. Then I realized that I had inserted an SD card and initiated a backup…. which it might finished but I could not tell for certain (no notifications or progress bar until I rebooted my Chromebook).
In then end I had 650 files that needed to be deleted.
Turns out that there is a backup folder in my Google Drive that, when deleted, removed the files from the offline folder. The offline folder is some sort of shadow directory. Once I deleted the backup folder the files in question were removed from the offline folder.
Hope this makes sense.