The high ground? I would imagine high end drones that anyone with $$ could get would be more of a threat.
And if the FBI is making such a public deal of the high ground/observatories near White Sands, would this not trigger satellites from other countries to focus on the region?
You should try flying a drone anywhere around that area, and see what happens.
As for foreign satellites, anything is possible. Without knowing what is being tested at White Sands, and how, we have no idea what methods of monitoring would be suitable.
If you want a reason why the FBI is temporarily shutting down a solar observatory (started by the DoD, ironically), I am giving you the most plausible answer, that's all.