Moffat claimed the clue lay in something Sherlock did that was out of character. I believe the girl screamed because her abductor was impersonating Sherlock at the time which certainly implies there was a Sherlock mask/duplicate coat around the place, though.

Sherlock's insistence that John doesn't change his position is a canonical reference to The Dying Detective, incidentally. In that story it was so the signs of faked illness on Sherlock's face couldn't be detected by John getting too close, so that might be something to do with the figure on the roof not being Sherlock.