Still a mystery to me. But you case #1 is really the system (OOM killer) zapping Gimp without warning.
I've wondered if scenario #3 might be a case of data remaining resident in memory after the crash, and perhaps Gimp recognizing the data as it's own after the (application) restart? Obviously whatever it is, it is intentional, or there wouldn't be a popup message to address the scenario.
Thanks, just the same.