The weird thing is I was told in the past that Zipcar's telematics module would retain the last 'active' card when it's out of cell coverage, plus has a fixed set of 'local' cards programmed to it (eg, for fleet service).

There is also typically a 'local' card available in situations like parking garages where the Zipcar is valeted up and away from the public. It'll always start the vehicle session, signal or no.