Sherlock: 20 Easter Eggs, In-Jokes And References
14. Vatican Cameos
In both A Scandal In Belgravia and The Sign Of Three, Sherlock calls out the strange phrase Vatican cameos to get Johns attention. In the latter, John explains that this is their code for "battle stations" in other words, someones going to die.
This is yet another callback to an undocumented adventure. This time in The Hound Of The Baskervilles, Holmes refers to a case he calls the little affair of the Vatican cameos, which apparently involved the Pope!
Quite why Sherlock and John use this as a battle cry is unknown, though several sources on the internet say that the phrase was actually used in just the same way in World War 2. But this should probably be taken with a pinch of salt.