When spy fiction shows someone "sneaking" in or out of a place (like a cat burglar), isn't that completely fictitious? I thought infiltration was always "social", like being in plain sight in a disguise/cover/assumed identity.
Are you implying that no one has ever snuck into a building to steal or kill or gather information? "Completely" and "Always" are pretty strong modifiers. And what era and situation are you talking about, and where?