Sherlock aspires to be more mechanical, but his emotional nature gets the better of him, especially around his brother, to whom he acts petulantly, as if he were a child.
Mycroft aspires to be more emotional, but his mechanical nature gets the better of him, except around his brother, to whom he acts protectively, as if he were a child.
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!
It’s also canon! “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.” —HOUN