Sherlock emerges from the kitchen, pill bottle in one hand and glass of water in the other, in time to see the two men exchanging farewells at the door. He sees the physical therapist's expression as he puts his hand on Ted's arm, and it strikes him that there might be more than mere friendliness with a patient there. He would even go so far as to say that there was attraction there. Curious.

Then Ted turns around, and Sherlock blinks at the expression on his face.

He hadn't picked up on any suggestion that Ted might be something other than heterosexual, but he's only interacted with him in conjunction with Joan, who Ted is clearly in love with and attracted to.