Holmes is smiling despite himself as he makes his way back to the bathroom and Watson. If Watson's words are a deliberate attempt to appeal to Holmes, either he doesn't notice through his flattery or he'd rather not notice at all.

"That is a very good point. Why advertise when the product is not for sale?" he says as he enters the bathroom. Walking around naked produces such a charming sensation of intimacy and companionship, and when he enters a room to discover Watson's naked form, it's difficult to hold back. He gives Watson's arse a light smack, grinning at him, and he lights himself a cigarette.