'Vatican Cameos' is a phrase used 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 to 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.