Well if the manager on shift had received no training on the alarms in the building that might work.
But who would install a silent alarm on a gas detector? The point of such alarms is to alert the occupants of mortal danger. I have smoke alarms with CO detectors in my house because I gas heat. They have audible alarms.
But like I said this is a display of total lack of critical thinking.
I think that's a safe bet.