In WWII the phrase “Vatican Cameos” was used when a person who was not in the British army came before the general, or other high up ranks, as a signal to the other officers that the person was armed.
So when Sherlock says “Vatican Cameos” to warn John that the safe has a gun in it, it’s not something that they’ve set up as a code word- It’s a code Sherlock knew John would know- being a soldier!
sorry but I’m skeptical about this. OP did not cite his/her source and I can’t find this fact anywhere on the internet. it’s just people from the sherlock fandom making the statement without reference on every website. also, what are the odds that a phrase ACD used in the early 1900s in HOUN would be used forty-ish years later in WWII:
“I must thank you,” said Sherlock Holmes, “for calling my attention to a case which certainly presents some features of interest. I had observed some newspaper comment at the time, but I was exceedingly preoccupied by that little affair of the Vatican cameos, and in my anxiety to oblige the Pope I lost touch with several interesting English cases…”’
it’s a very specific kind of phrase. also, steven moffat says that he based the scene off the above canon quote that mentions the vatican cameo case. I think he would have mentioned it being a WWII codeword because steven moffat sure likes to look clever.
“Steven: I imagine that sometime during the Vatican Cameos case [not depicted on the show] he had to tell John, Dr. Watson, to duck. It’s a rather obscure moment that everyone’s fallen in love with.” x
once someone shows me a source, I’ll believe it! I think that’d be pretty awesome if it were true.