I can't find a link, but I was sure there was something in pre-Check that made it clear that you still could be 'randomly' selected at any time for a higher level of screening.

I personally witnessed at SEA once. I was in the regular lane, and every single Pre pax was swabbed and bag checked while I was waiting for/getting my grope. They only swabbed hands and the bag check was pretty cursory (right on the belt), but there were some surprised and not happy people.