If the US military couldn't recover it with the legal right to be there and no concern about having to be clandestine, I doubt a terrorist group will successful

If the US military couldn't recover it with the legal right to be there and no concern about having to be clandestine, I doubt a terrorist group will successfully recover it. Probably the Air Force decided it would be cheaper to keep the site under constant surveillance for a few centuries than to go through the nightmare of trying to excavate a swamp well enough to extract the thermonuclear stage.