At one point during the movie Avengers: Age Of Ultron Tony Stark exits his Iron Man armor, putting it into sentry mode, and explores the next room suitless. The thermal scan performed immediately prior suggests the suit can sense humans (or the absence of them) in the adjoining room Tony goes into. Otherwise I expect Tony would have continued to wear the suit to go into the next room if he wasn't already certain of its lack of danger.
Yet the suit doesn't warn Tony or attempt to intercept two people who approach him from behind.
Is the suit unable to detect these people? Is the sentry mode faulty, or is it not as capable of detecting people as it appeared earlier?