Whenever I read about such organizations I always wonder who cleans the toilets. Whether they are real like the CIA, Five Eyes, or even Dr. Evil someone low down on the ladder has to get access.
That said, in the US, the janitor is probably a contractor. Typically ex-military, they have to pass a background check and are cleared to work in public spaces. When cleaning secure areas, they're escorted and watched.
Even if you're assigned to a NATO joint command you're still paid by your country's military.
This goes back at least to the original "social engineering" that hackers used to do in the 80's.
In more recent decades, it was also a later well-publicized way that the press knew about the impending starts of the Iraq and Afghan wars.
[1]: https://www.washingtonpost.com/wp-srv/politics/special/clint...