They have to do manual pat downs for people who "fail" the scanner, either because there was something that warranted a double check or the computer randomly as

They have to do manual pat downs for people who "fail" the scanner, either because there was something that warranted a double check or the computer randomly assigned someone to receive extra scrutiny. I've walked out of the security perimeter to reschedule after a flight cancellation in an airport with <10 passengers in the terminal. Upon reentering I was "randomly" selected for an explosives swab despite passing the first time with the same bag and not walking more than 50 feet away from the TSA checkpoint.