"Random" alarms in the pre-check line are now happening at SEA. I observed it happening at checkpoints 3 and 4 (not sure about the other CP's but my best guess is that its going on throughout the airport. Can't confirm that though). When the random alarm hits, they are directing people through the NOS
Also, I've seen the alarm go off on two people in a row at PDX. So if the person in front of you gets the random alarm, don't assume you are necessarily safe from avoiding it.