Oh thank you! 4laugh Any fan of BBC Sherlock is a friend of mine!
I get this question quite a lot actually... The item is "Tourist Hat" created by reapersun. She made it back when they were filming Season 3 of BBC Sherlock. It was sort of an announcement celebration. You had to decipher a code, it was bonkers. But there's plenty of them in the marketplace. They should still be pretty cheap.
Now, you HAVE to be wearing the hat on your head for the item to come up. You also must have your avatar clenching their fist on the far side for him to appear (since he's quite the gentleman and won't show up otherwise, haha). Have fun with it!
There's also another BBC Sherlock item that reapersun created for the airing of Season 3 called "Mournstache".