The device is simply fun as going on when you have found ne power line or the like, and in my opinion it is relatively accurate because vertically above and below light switches energized power lines are present and they are alarmed precisely to 2-3 cm, until 10-15 cm to get in which only a few red bars appear in a safe area.
Unfortunately, such devices always even the smallest parts are detected and displayed so you should think with a bit if you go to the search. This sensor hole up under the Bosch logo works but apparently very accurate and can thus even put a mark or see.
