An image is stored in a P&T Assets field and has been working fine. An editor emailed me saying when she opens an entry to edit it, the image is gone. I tried to edit entries and it worked fine. I even logged in as her and used the same browser and it worked. I then watched her edit it on her computer (Win 7 with Firefox) and sure enough, the image is not there when she opens the edit page.
I made sure she was using the right url, https (and not http which caused problems), no domain mismatches, etc.
A strange problem to start with, the inconsistency makes it even more 'fun'.