"In late summer 1943, a small storeroom on the main corridor of Churchill’s War Rooms was fitted with a new door. It sported a lavatory-style lock and its appearance explained the construction work that had been going on in the room for the previous couple of months. Churchill, it seemed, had been given the luxury of a flushing toilet. A passing secretary may have felt a moment of slight envy (all the other workers had to choose between foul-smelling chemical toilets underground or a trip up at least two flights of stairs), but would otherwise have given the door little to no thought. But when the lock on the door was switched from ‘vacant’ to ‘engaged’, it didn’t mean that the prime minister was answering a call of nature; he was instead making use of a secure radio-telephone link to talk directly to the president of the United States of America. It was perhaps the most secret communications facility in the world, but there were no armed guards, no security passes – just the clever misdirection afforded by that simple lavatory-style lock." Source: https://www.historyextra.com/period/second-world-war/churchill-war-rooms-london-history-secrets-behind-scenes-visit/