You ain't gonna believe some snot-nosed Street Arab over one of your employees, are ya, Sir Regis?
Sir Regis: (stammering) I, uh...
Mycroft: I applaud your noble intent (said with sarcasm), however, you were severely flawed in your intent. (picks up the mask) Look closer at this mask. Feel it. It's pure leather, highly endurable and, as I believe it's phrased, "tough". Look at the ends, were it was torn off. It was a very violent rip, which would only be accomplished by high adrenaline. However, after such a burst of energy, it would require some time for the arm to relax into it's average position. This poor boy's arm is hardly stiff enough for such a feat. Also, look at his gloves. Such a violent act as ripping off a mask such as this (points at it with chubby finger), which, as I've just described, is pure leather, would leave quite a burn mark on any hand-wear, and yet his gloves are clean. I presume he hasn't changed them since you apprended him, has he, guard?
Uh, no... (glances around, everyone shakes their head frantically)
Mycroft: As well, there is some mud on the front of his ankle, but not the rest of his boot. This supports his story that he was tripped, and finally, that deluded woman in the expensive dress only saw the back of the thief's head, and was obviously so traumatic she would believe anything thrust upon her.
Sir Regis: (stammering) A-a-amazing! Astounding!
Mycroft: Elementary. Utterly commonplace. (offering pudgy hand to the boy) Mycroft Holmes.