This upcoming meme originated from the BBC show "Sherlock" a contemporary remake of Sir Arthur Conan Doyle's Sherlock Holmes detective stories. A summary of the third episode of the second series follows:

"Moriarty plans to frame Sherlock and expose him as a glory-seeking fraud who actually committed crimes and then pretended to solve them. He hacked several computers and created a new identity for himself – as an innocent actor who was hired by Sherlock to play the role of Moriarty, a fake villain who Sherlock blamed for multiple crimes. In the finale, Moriarty told Sherlock that if he didn't commit suicide after confessing he was a fraud, Moriarty's people would kill Lestrade, Watson and Mrs. Hudson. So Sherlock made a phone call to Watson and confessed to having masterminded the whole scheme and admitted he was a fraud, and then jumped off a building, presumably to his death. Watson never believed him. At the end of the episode, Watson visited Holmes's grave and told him he still believed in him. Holmes, still alive, watched from a distance." – summary from yahoo answers ( because I'm lazy.)

In response to this episode fans have been creating graffiti and artwork around the phrases "I believe in sherlock holmes" and anything else they can think of. It's awesome.