> The NLJD antenna head is a transceiver (transmitter and receiver) that radiates a digital spread spectrum signal to determine the presence of electronic components. When the energy encounters semi-conductor junctions (diodes, transistors, circuit board connections, etc.), a harmonic signal returns to the receiver. The receiver measures the strength of the harmonic signal and distinguishes between 2nd or 3rd harmonics. When a stronger 2nd harmonic is represented on the display in red, it indicates an electronic junction has been detected. In this way, a hand-held ORION is used to sweep walls, objects, containers, furniture, and most types of surfaces to look for hidden electronics, regardless of whether the electronic device is turned on.

TL;DR: You can easily detect it while it communicates via GSM, and the device is also shielded quite badly, resulting in lots of easily detectable RF interference while it works.

All you need is a cheap RF detector. Having access to a full spectrum analyzer or a SDR will make this even easier.

All this gets much harder while the thing lies dormant, waiting for noise activation or commands. So the "quick bug sweeps" you see in the movies are more difficult.