The damned book. Sherlock's interest snags on what Dorian is carrying--and it doesn't take him long to snap to the conclusion that his 'phone' and Dorian's 'book' are functionally the same object. Now that's new. And fascinating. So something is creating communicators according to the apparent needs of the subject--now, whether or not the subjects agree--

"May I see?" He holds his hand out for the book with the obvious faith that Dorian will, indeed, let him see. "Here, here's mine." He offers his tablet phone with his other hand, as if turnabout is fair play.