Tracking it back: the phrase last had a spate of usages in 1988 about the National Security Agency, command center of our global eavesdropping. Once gathered, wrote Peter Iseman in The Times Magazine, voice and signals intelligence must be analyzed at Elmendorf Air Force Base in Anchorage, then at a huge underground facility hidden beneath a pineapple field in Kunia, Hawaii, and finally 'behind the Green Door' of National Security Agency headquarters at Fort Meade, Md. (I once got lost going to an interview there and asked a local cop where N.S.A. was. You mean No Such Agency? he said and pointed the way.)
During the Iran-Contra investigation that year, an N.S.A. official testified that Gen. William Odom, then N.S.A. director, had ordered him to get an employee who had assisted Oliver North out of a public job and put him behind the green door of N.S.A. as quickly as possible, which meant out of contact with outsiders.
Before that, the phrase crashed into the public consciousness in a sense
unconnected to espionage. In 1972, Behind the Green Door was the title of one of the first pornographic movies widely released in the United States. (Deep Throat