He jolts in place turning to look at the... twin here. Arthur and Sherlcok. Arthur who is Sherlock who is Arthur. He gapes dumbly at the both of them, watching as the Detective simply continues about like nothing and Arthur lets it happen. His gaze follows Sherlock has he walks off only to then go back to Arthur.

"Not how you planned?!" He splutters. "Were you going to tell me?!" He's a little worked up. If he were thinking rationally, he wouldn't have expected to be told. But still.