That's Lucas Oil Stadium where the Indianapolis Colts play. That specific picture is from Super Bowl XLVI which took place in 2012. There was another sniper and spotter in the opposite corner.
But as others have said, most major events, including every regular NFL game have snipers in attendance.
They also have cops dressed up at fans to see if they can get through security checkpoints. They'll have the wrong ticket, play dumb, or try to sweet talk guards to get to places they shouldn't. An IRL pen test.
It's a landing of a stairwell in that picture.