You saw it in "A Scandal in Belgravia": Sherlock's bedroom, a pretty sparse place apart from the periodical table framed on his wall, and some other sort of frame over his headboard.
"SHERLOCK" fandom is such that no sooner than the day following the broadcast, fans had already figured out what was hanging on top of Sherlock's bed. It's some sort of diploma written in Japanese (see here). As a Japanese, I was able to make out that it was a certification for Judo (not "baritsu", as Conan Doyle put it, but properly Judo).
And then this was followed by the very astute observation by Tomoko Koide (@lutenist_koide) that the name doesn't say Sherlock Holmes (it says ジェフリー・ディ・ベイシュ, Sherlock Holmes written in Japanese looks like シャーロック・ホームズ）. Moreover, and perhaps more importantly as clues go, the date on the diploma says, Showa 40 (昭和40年）, the fortieth year of the Showa emperor (Emperor Hirohito)'s rule, which translates in the West as 1965.
We were wondering where the arts/props people had gotten hold of this judo diploma, when Tomoko found this amazing link....
The investigative powers of SHERLOCK fans are such that someone has already figured out that this diploma actually belongs to "Jeffrey D. Beish" of the US Air Force. You can see a cleaner copy of his rank certificate here. So, did the arts/props people on SHERLOCK find this page online, copy the certificate photo and do all sorts of things on Photoshop? Or. Or, was it Sherlock himself who made this to pretend he's a judo black belt...hmm....
Anyhow, as Mr. Beish's web page says, the diploma is a certificate from Kodokan, the official body that oversees Judo, and gives Mr. Beish a Shodan (first rank), allowing him to wear the black belt. Brilliant.