Story time. They tested the original F117 stealth fighter by flying it behind another decoy aircraft, telling everyone involved in the test that midway through the overflight, they would switch on their magic device and the aircraft should disappear from the radar scope.
Well, the plane flew over....they counted down to the 'big moment', threw the switch....and nothing happened. "Sorry, sir, looks like your magic box didn't work," chuckled the guy at the radar scope. "Yeah, bummer," said the aircraft team.
LITTLE DID THEY KNOW....flying not too far behind that decoy aircraft was the F-117. It never showed up on the radar at all. If it had, the radar would've shown 2 planes.
THAT is how you run a test of a secret project where the third party testers (who you need) have no idea what they're testing, and whether it even worked. Genius.
For an amazing read, check out Skunk Works by Ben Rich, it's PACKED with stories like that, and many others.