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.
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.