They physical fact of the base is essentially impossible to hide. Anyone with a camera can “out” the existence of the base, and its overall purpose, in minutes. The secrecy part refers to the actual purpose of the base, and of the drones housed there.
It's hard to imagine the ambassador of one sovereign nation going up to the leader of another sovereign nation and saying "we'd like to rent some space to house a spy agency and assassination squad right here next to your capitol."
Yet that's exactly what we do.