Most of the displays consist of graduated lights - green,yellow,red - like your stereo amp. The point at which the audible warning sounds can be varied along the scale. I suspect that it is now normally set at its most sensitive, ie. the first green light. Which means of course that the light display becomes meaningless and the screener <insert applicable noun> only react to the noise.
Suggestion: station the national guardsmen so as to guard the plugs. More useful than their current role of looking 'ard and occasionally bullying passengers.