In the Sherlock Holmes
books it refers to one of Sir Arthur Conan Doyle
's stories, in which he refers to another case Sherlock worked on to do with some Vatican Cameos.
In the case of BBC Sherlock
it is code for duck, as in get out of the way. It probably refers to another case Sherlock
worked on not shown in an episode, in which only Watson knows to duck out of the way of, in this case, an oncoming bullet. Most usually used by Cumberbitches