I did hear from support and it did resolve the issue. See below...
There's a few things that could be causing this behavior, but after reading your description, I think the first thing to check is that there isn't a file stuck in the temp/edit folder. It could be the case that a folder isn't deleting from within there, causing the document to remain locked.
If the user is editing the document, perhaps there is still a lock being held onto from the machine they're on.
You can check %temp%\Edit from their local computer, to see whether there is still a folder here. When documents are edited, an entry is created here which stores the temporary file, and this normally should be deleted once brought back into DocuWare.
If this is still there, then this could be the reason for the lock being persistent despite the user closing and shutting down. You can safely delete anything within the temp and edit folder.