According to The Drive, even a small number of civilians who have access to the famous "Area 51" facility in the United States are required to use special Foggles glasses when visiting it, limiting their view. The windows of buses running here have similar properties - they are not transparent. Everything in Area 51 is surrounded by a veil of secrecy, which is why some rooms have no windows at all.
For reference: Area 51 is a secret remote division of the US Air Force Base Edwards, located in the south of Nevada, 133 km northwest of Las Vegas in the vicinity of Salt Lake Groom Lake. Presumably, tests of the latest aircraft are being carried out here. Flights over Area 51 are strictly prohibited.
The US Air Force is not the only organization artificially limiting visibility at classified sites. So, in the demilitarized zone of South Korea, visitors, looking through the installed binoculars, can only see what is in the distance. The zone itself cannot be seen through it.
The idea of limiting the physical vision of visitors to such objects arose relatively recently. In the case of Area 51, it was decided to use Foggles, which pilots use in training to simulate poor visibility. Another function of this device is that the pilot with Foggles sees only the dashboard, and the image of the rest of the environment is blocked. Thanks to this, he is able to focus on the readings of the instruments.
The increased secrecy of objects always attracts attention, so the use of visibility limiters in Area 51 is perfectly combined with the closed nature of this object. It turns out the following: a visitor to the zone, wearing Foggles, sees only what is allowed for him, and the pilot, who is on the simulator, does not see the visitors who are looking at him.