While it might work I don't think I'd rely on it without doing an awful lot of experimentation. A relatively powerful and continuous source of RF would seem likely to register. I've heard of people using these to pick up police or security frequencies - fairly powerful transmitters within very short distances (50 meters).
A cell phone transmits intermittently - no way of knowing until you try, if it will see the cell phone. A low power, simple bug might or might not be powerful enough. Like a cell phone, it would seem that a tracking transmitter would be intermittent - to preserve power and to avoid detection. Some tracking devices don't transmit until receiving a command.