| A secluded or unobtrusive building or apartment considered safe for
use by secret agents as a base of operations, as a refuge, hideaway or for a
personal meeting. Archetypically, out in the middle of nowhere, with generator, cistern, and the like, it can sometimes simply be a city apartment that keeps a very low profile. "Oh, the Smiths? You hardly see or hear them..."
Alternatively, a legendary night club/discotheque in Milwaukee, Wisconsin, behind a door marked "Universal Imports", and otherwise unmarked and NOT in the phone book...hmmmm.... | 
