Seemingly they can kind of easily rummage through whatever, which the Snowden case gives hints at. This guy supposedly had briefing slides from the Chairman of the Joint Chiefs of Staff. You would think that's near impossible, for someone at his level, but somehow it wasn't.
For example, if you've consumed cannabis in the past 12 months? Forget about it.
Source: my interim access to Public Trust (several tiers below the type of information in these leaks) was revoked and final adjudication took 5 months because in November I admitted to partaking 4 months previous to filling out my first form.
I wont be surprised if they actually have some knowledge mgmt model that shows the point above which they cant keep tabs on everything. But its the govt. They are never going to admit that in public.