If a file is open when you mark it online-only, the eviction fails permanently and silently. There is no way to see which files failed and no way to re-trigger them, so on a large account they accumulate until a substantial part of the disk is held by files the user already told Dropbox to remove. In my case 1.39 TB. The usual culprit is QuickLook, invoked just by browsing the folder in Finder, so simply looking at a folder can permanently pin files. Please queue failed evictions and retry them when the handle is released, and show which files are still local against the user's wishes.