It's hard not to smile at having some information he knows Jim desperately wants and that he simply won't provide. A turn of the tables, but at the end of the day, just another childish game of keep away with a psychopath. In spite of this, Sherlock knows he'll need to gather his allies together soon. He doubts Richard Brook will make another appearance; Moriarty doesn't do repeat performances. That doesn't mean he won't need all the people he can trust behind him anyway. The last time Jim was here, he'd barely told a soul. He knows now that won't work if he wants to survive. If anyone does.

Several people have started to give them odd looks, so Sherlock finally releases Jim's hand, his own stinging from the pressure. He has noticed the gun, and he doubts Moriarty will pull it here, but he knows exactly what gun it is, and that it has yet to fulfill the purpose he saw for it.

"What are you doing here?"

He doesn't refute the promise. Sherlock knows the lengths to which Moriarty will go to keep his word, whether he liked it or not. It's best for now to focus on the present.