[baybal2]

I don't think one can cover just anything in an open society. Even the truly secret military gear like the original H-bomb, F-117, A-12, anti-powerline bombs, and navy stealth helicopter were impossible to hide completely. At least the fact of their existence was out long before the public saw them.

Even in paranoid USSR, people knew of computer guided recon drones, spy sats, Shkvals, Dvinas, R73 and Strelas — all of which were more secret than the bomb, and were equally bordering on "sci-fi" and unbelievable to the layman at the time.

The leak of Shkval's existence was of particular embarrassment to the Union's military and counterintelligence. Supposedly, there were less than 50 people in the whole Union who were privy to info on them in full, but even before it entered service it became an "urban myth" among civilians in military cities and such.

[narag]

I don't think one can cover just anything in an open society.

If you really want to apply common sense to the hypothesis that aliens are real and here, consider the possibility that aliens themselves have become much more careful since the fifties, just because they know we have more tech to detect and intercept them and governments are no longer able to hide anything.

Or maybe they have a spy net on the ground already.