The nods given by Arthur Conan Doyle's original detective stories in BBC's "Sherlock" are always fun to explore. Now, another meta reference has appeared in "Sherlock" season 4 episode 2 titled "The Lying Detective." One eagle-eyed fan has spotted an uncanny Easter egg: a letter from Torchwood.

The episode had a scene in which John Watson discovers a package containing his wife Mary's DVD delivery to Sherlock. Carefully set up on the mantelpiece, among the detective's "things he can't figure out" (as Mrs. Hudson puts it) is a tiny postcard where the Torchwood "T" logo can be seen. A fan on Twitter posted a photo of the Easter egg, which quickly prompted several "Bring back Torchwood" replies.

The shout-out to the alien-busting Torchwood is rather odd, but Jack Harkness did warn Holmes of Watson's baby who, according to him, is an ET. "Sherlock" season 4 had been wading on the waters of sci-fi, referencing Doctor Who on the promo image before, reported Doctor Who Watch. Tucked in the shelves is a book written by Lavinia Smith, a woman who "Doctor Who" fans will recognize as the loving aunt of Sarah Jane Smith, whom she brought up after her parents were killed in a car crash.

Going back to "The Lying Detective," another letter can also be found on the mantelpiece, addressed to S Holmes at 156 Montague Street. In the original Sherlock Holmes story "The Gloria Scott," it was revealed that the sleuth first lived at Montague Street before moving to Baker Street. Whatever that letter encloses could only mean it's something Sherlock could not himself decipher.

Anything on that mantelpiece could hold the key into defeating the elusive Euros -- and maybe answers will be given on the next episode, presuming Baker Street stays in one piece. "Sherlock" season 4 returns January 15 with "The Final Problem" on BBC.