Trying to keep a "detain"-button secret, having it manned by thousands of people that only have a high school diploma OR a year of work experience in security/aviation/screening, doesn't sound easy (those are by the way the education requirements for a TSO according to usajobs.gov). Since a leak could stem from a minor overstep of their contract (ohh, I told a buddy about it, big deal right?), and there are frequent policy violations by agents, it does look grim for the existence of such a button.

I heard of them kicking the machines to make them go off though, so there's a "button", and it's public knowledge that they will use it if they want a pat down.