[skibbityboop]

Great question, and yes, some of the better devices implement a small buffer so may be able to keep a day or even two in memory while they're out of cell coverage. If you activate the SIM you might get a small data dump.

GPS doesn't need a SIM, and indeed the devices will have a fix all the time whether activated or not. They do require cell service to send their updates anywhere. They're all simple GPS receivers, not transmitters, thus require an active SIM.