USAF/Brian Shul - Shul, Brian (1994). The Untouchables. Mach One. p. 115. ISBN 0929823125.- Alternately, the best selfie ever

Designer Lockheed and the Air Force treated the project with intense secrecy, and when still totally classified, the CIA recorded any hints that reporters, analysts or civilian plane watchers might have as to the jet’s existence. It didn’t matter who suspected it. If airport cab drivers were spreading rumors about secret doings at Lockheed, the agency wanted to know.

It is pretty amazing how much industry insiders were able to figure out just based on a few seemingly unconnected data points. Just from seeing which vendors were visiting Lockheed, the fact that Kelly Johnson was hard to get a hold of and the continuing development of an air to air missile after the only known platform capable of launching it was canceled was enough to give some insiders the information they needed to determine that Lockheed was working on something special indeed. Not only were some insiders able to figure out that Lockheed was working on something special, they were able to make some pretty good guesses regarding its capabilities.