Something akin to satisfaction shows in Mycroft's face, layered with other things like curiosity, approval, amusement, and (if you're looking for it) playfulness. The emotion comes through mostly around the eyes, and the effect isn't overly warm, but it's a couple degrees above what most people will ever see from him.

"I can tell you that your assumption regarding my identity is correct, Mr. Shore," he says, then gives a hint of a smirk.

"Presumptuous, indeed."