The seats behind the podium are VIP seating. The secret service would never allow any individual in disguise to be that close to the President. Unless he had been authorized by someone above them....
Back on a serious tip. I bet Secret Service screens signs that people behind him are carrying too. Don't want any embarrassing ones showing up. Eh?
I'm coming down on the side that this guy was purposefully put there with his wighat, Snowden glasses, and van pic as part of the Q/Snowden thing.