I agree, that would be interesting. Removing the SIM would not prevent the phone from communicating using GSM (emergency services, should still work). But I guess it would probably prevent data (EDGE, 3G, 4G etc.) from being active. Also I guess the internal processor in the SIM is fairly low frequency (5 MHz I think) so the detector will probably be more sensitive to this than it is to the internal CPU in the phone.