There have been TSA personnel on here, as well as others who have observed the WTMD in action, stating that there is a pseudo-random red light that shows up on the side of the WTMD to indicate a pseudo-random secondary. The light is, supposedly, smaller than the area alarm indicators. I cannot find any of the other posts about it at the moment, but those are my recollections from the times this has come up over the past several months.