John had more than a little experience with stolen goods- an ashtray from Buckingham Palace, for example. This was one deduction he didn't need Sherlock for.

"Nicked it, didn't you," he accused brightly with a conspiratorial grin- unwrapping it completely and admiring it- knowing Howard hadn't mortgaged his soul for it made the gift far easier to appreciate. "It's really- what's this on the back? Oh, nice... would be pretty handy if they'd let me have it..."