Since the mystery pax did board the plane, his bp must have been scanned. By a process of elimination (easier if he really was in fc), they should know his name. Does that make the plane more secure is a different story. [IF TSA is as vigilant as they want us to believe, what could he have placed in the [loo] restroom that would be a threat.]
Did pax have checked luggage?...knowing his identity could help.