Which gets at the real question.. what is the scanner actually checking?

From the numerous comments here, it's not matching the boarding pass to the flight or gate. Is it just boarding group? Is it that "yes, this is a valid ticket" or something else entirely?

Seems like an easy way to "disappear" - basically book a fight going somewhere, look around until you find a flight with open seats (Southwest would be best as no assigned seating), and then... poof?