I've seen buildings where the buttons are 'burried' in a few mm 'deep' the wall, and painted over so that you don't notice them at all, the wall looks really sm

I've seen buildings where the buttons are 'burried' in a few mm 'deep' the wall, and painted over so that you don't notice them at all, the wall looks really smooth. The guards know where exactly the spots are, you see them waving their devices in a seemingly flat/blank wall, then the beep sounds, and they walk towards their next 'random' spot.