This isn't fool proof but the FAA policy that my dad experienced years ago was that the passenger was dissuaded from leaving, the captain came out, and he was told that if he exits the plane they have to sweep it and because of that he'd be denied reentry. Passenger insisted. Argued a bit.
Long story short, the passengers all had to disembark with their luggage and go back through security screening. And the flight crew presumably did their standard pre-boarding sweep.
This was, oh, 2002 or so. I wouldn't be surprised if they even let you off the plane anymore, at least not until the 3-hours are up that the new regs mandate.
And similarly, there has long been double-checks to make sure that no checked luggage is on a flight that isn't matched to a passenger. My understanding is that luggage that is freight checked (like, they lose your luggage and throw it on a later flight to get it to you) has to be more thoroughly screened.