Some do use noisemakers and special .223 paintball rounds that work in the patrol rifles with a special bolt to allow the rifle to function semi-auto with the reduced recoil of the paintball rounds.
However, I can’t see an exercise happening without all LEO knowing it’s going on. Typically, many surrounding depts that would be providing assistance for the real thing will be invited to come and train so Officers can learn each others buildings, some are real mazes.
Active shooter excercises are part of a national drill. There is testing of the alarms and the response of personnel to the alarm. It does not involve noisemakers. Anyone using noisemakers is doing a drill apart from the national drill or doing the drill wrong. At my location the alarm went off at the wrong time and completely screwed up the drill locally, but the fault has been identified, so in that sense the drill was successful.