HUGE Spoilers... Well, worth a read, anyway... Went rounds on Reddit. Figured out almost all of this, and then someone got on and pointed me in the right direction. Great theory. Explains the only thing I couldn't understand. And takes it back just a little bit farther.
(gif) His expression is so hard to read! If he were a normal person I would say this looks like anxiety, perhaps disappointment, maybe guilt or regret. But he's not a normal person. He's Sherlock. For all I know this expression could be "Ah, I think perhaps I'll have the beef stew."