They showed an agent walking through the hallway, calling into the radio the floor was "clear". Directly behind that agent, the sniper entered the floor and the room.
I agree the counter snipers (who should have been on various rooftops looking with binoculars at other locations) should have noticed a single window opening all of a sudden.
At the same time, we don't know how deep this treason goes. 1) Someone leaked the plans to blow up the Capital 2) The sniper who took the shot was a secret service agent (had the pin and the radio earpiece).
I don't think the sniper was an ACTUAL secret service agent, but rather posed as one. The sniper was the same dude as poisoned the terrorist, while posing as a military guard. He likes to pose as lots of things, apparantly. Of course, we are not going to get the question answered as to how either of these things was remotely possible. LOL.