She's babbling again and he wishes she would stop; he has work to do, and time is ticking out.
"You look sad," she says, and that gets his attention. "When you think he can't see you."
He looks up from his microscope, spares a brief glance for John across the room. She's deducing him, and it's as if he actually sees her for the first time.
His brain stalls momentarily, trying to process the new information. Is that how longing feels, he wonders?
He, Sherlock Holmes, doesn't know. And yet he's prepared to trust her with his life, and more.