So, idk how familiar you are with Battlestar Galactica, but this guy, Leoben, is a cylon, which is the race of machines that cause trouble in BSG. However, Leoben looks and acts humans so it's very difficult to tell that he's a machine, and Sherlock shouldn't be able to pick that up right off the bat.

He can, however, determine that something is off about Leoben, and that he's withholding information about himself. I'm sure we can work something out.