The Pentagon had stuff they NEEDED to keep secret...like at Groom Lake/Area 51... they would have lied, or at least dissembled.. when asked about odd sightings of aircraft.

My uncle did blueprints for Consolidated Vultee in San Diego in the 50s when the VTO planes were first revealed. He told me he'd known about that for several years... and that I wouldn't believe some of the secret stuff they were working on.
   Of course, that sort of stuff is still going on. There will always be secrets... but we 'usually' hear about them later.. like the Stealth stuff.
   Alien craft hidden at Area 51?   *shrug*.. that assumes that alien craft which can cross interstellar space can crash and be found and no real leaks about it... I am still waiting.......