I would suggest that tracking former employees or contractors that had security clearance and did not misbehave while working for the NSA would be a pretty low

I would suggest that tracking former employees or contractors that had security clearance and did not misbehave while working for the NSA would be a pretty low priority to keep track of - and fairly numerous. Particularly since any information they did have that was sensitive would generally lose value over time. I would expect that most of the NSA's resources directed at specific individuals are directed at individuals that they have reason to believe are up to no good, not just tracking low-risk folks for the sake of tracking them.