This is a side of Sherlock very few people have seen. Exactly the same people that have seen him sleeping, in fact, up until now.

Usually he is paying attention, analyzing everything. Usually he cannot stand to miss a second's worth of the world as it goes by. Usually, even playing the violin with his eyes closed, you would get the sense that he was watching you, and you'd be right.

Now, when he opens his eyes and looks at Cal during a slow section, he is genuinely surprised to find Cal looking back.