Apparently, much of the testing for the YF 12 was done at Area 51. The early test prototypes were tested at an outdoor wind tunnel. The test planes were unpainted and raised into the air for testing by a hydraulic lift. Testing was usually done at night.

The author concluded that some, if not all of the UFO reports from the area were just people stumbling across this weird aberration of a shiny silver disc-like structure hovering in the air in the middle of the desert.

No idea if true, because the book was full of some wild conspiracy stuff, but an interesting theory.