If you asked me, i would say what they use is an Electronic Wall scanning kit.

http://www.amazon.com/Zircon-59243-Electro...g/dp/B00009K76J

You most probably would have come across contractors carrying these around to check for live wires behind walls (before they drill etc).

These kits (esp the good ones) are capable of detecting even very low levels of current without actually having to come in direct contact with it. Put it up against a car window, and there is a high chance it will detect the small amounts of current that the laptop uses to keep the laptop running (most laptop users almost never "shut down" their laptops - its always on standy or sleep mode).

I might be wrong.