We all form our hypotheses about reality from our own experiences.
I understand that yours have been different from mine.
I held a high security clearance in the Navy, too. And, some relatives and friends had even higher clearances.
Certainly many folks have spoken to others about work related things in secret locations. But the ridicule, disinformation strategy works very well.
And, as numerous cases have shown, the government CAN keep incredible things secret for decades. It depends on a lot of factors, imho.
One person - who is convinced that aliens crashed in Roswell - told me the reason they kept it secret is because they are afraid the populace would panic if told the truth.
My response was - who would panic? Most people already think it's true!
Why on earth (pun) would NASA keep a discovery secret? Would would be the advantage? I can certainly see the advantage of trumpeting the news - I see none for hushing it up.