“The man with the key is king”, ~ Moriarty.
After asking Mrs Hudson if Sherlock’s ever, ‘had anyone’, i.e., ‘has he ever had any kind of a girlfriend, boyfriend, a relationship, ever?’, her answer, is, ‘I don’t know… How will we ever know…?’ Then we see John smile and throw his key up in the air. In an episode full of symbolic twirling, I think this is significant. We see Sherlock twirl Irene’s phone because he’s typing with her heart. Here we see John not know about Sherlock’s heart, even while he himself is the king of it.