Corrected entry: As the terrorists enter the building, they slide a flashbang up to the guard which explodes, and then the man is shot by one of the terrorists. About a half hour later when Sgt. Powell is dispatched to check out the plaza, he walks in the first floor and right up near the same elevator area, supposedly sees nothing, then leaves. We can assume the terrorists got rid of the guard's body, but there must have been other obvious evidence of a shooting/explosion in that hallway area (blood, wall or floor damage, remnants of an explosion) which Powell would have noticed.
Friso94
8th Dec 2017
Correction: Except that seeing how well-prepared and thorough the terrorists are, it's not unlikely to assume they would have cleaned that up to keep up the guise of nothing out of the ordinary going on as long as they can.
Friso94