Visible crew/equipment: When Tony is seen climbing out of the elevator shaft, when he's up there looking around a crew member's boot is visible stepping on a silver wire in the upper left hand corner of the frame. (00:38:15)
Factual error: Detective Bowden's son is ejected from the front of the car and there is a hole in the front windshield. The collision was a straight side-swipe and his momentum should have taken him out the passenger-side window. He would have gone out the windshield if the car had been rear-ended or if they had crashed head-on into something, but the car came to rest in an open field.
Continuity mistake: During the scene when the fire dept arrives it is raining heavily yet when they arrive inside the building to help they are all completely dry.
Factual error: At the beginning of the movie; a person is seen falling from a top floor high-rise. Pushed, thrown or jumping, his is highly unlikely. The exterior windows (those from floor to ceiling, or those that replace traditional exterior walls) would be triple plated, like that of a bank partition (think of 3 car windshields glued together) One would have to repeatedly bang on it with a sledgehammer before it would even crack, let along jump clean through.
Answer: There weren't any clues. It comes off as a total surprise during the reveal. But watching it for the second time, I did notice that she spoke very less compared to others and no one was suspecting her. Usually, that kind of character ends up being the bad guy.
Earthling