I find it a little disconcerting that the same people who are being trained to risk assess passengers in the security line are so adept at their job that they fail to notice a non-functioning metal detector.
I think it would be number one or two on the checklist of operating a metal detector.
1) Is your apparatus on and functioning?
2) If yes, proceed with passenger checks. If no, turn on apparatus.
Seems likely that you should know the status of the apparatus you are controlling. I don't sit at home looking at a black screen on my TV wondering why there is nothing on today.