'Vatican Cameos' is a phrase used in BBC's Sherlock when Sherlock is about to open Irene Adler's
safe. The phrase first originated in World
War 2. It was used when a non-military person, who was armed (
gun or knife) entered a British military base. The phrase was a signal for everyone duck out of the line of fire. Sherlock knew that John, being a military man, would recognise this phrase and duck out of the way of the
gun in the
safe. It is not a code phrase which Sherlock and John coordinated as a safe word. Very clever, don't you think?