Ghost of a flea

What makes a "Secret Base" secret? By "base", I mean those British Government installations or military sites you've seen surrounded by razor wire fences and guarded by Ministry of Defence (MoD) police. How on earth can these sites be secret? The UK Government hasn't (yet) developed stealth technology in the visible range of the electromagnetic spectrum. So they can't very well make, say, Faslane nuclear submarine base suddenly disappear as you come around the corner!