It's a different look for Sherlock, one Cal doesn't think he's seen before. Sherlock always looks like he has his guard up, just a little, even when - even in conversations where he doesn't necessarily need to.

(Cal doesn't mind. He usually keeps his guard up, too. It's safer. It doesn't always work, but it's safer when it does.)

It makes him look . . . something. Cal watches him, absorbed, trying to figure it out.