The army/airforce actively spread rumors of aliens to hide their activities.

Probably all of those, because it's really unlikely that aliens have come to earth and even less likely that an advanced civilization capable of interstellar travel, would ever get busted.

Military term for this is "broken arrow"

It happens more than you would think mostly by the US but also Russia. Usually they find the things pretty fast but a few are just missing and at least one I remember was underwater somewhere it couldn't safely be recovered so the area is patrolled.

I think one of these armed hydrogen bombs they dropped on NC was never recovered. Someone said that online I didn't read it in a publication so idk.

But there was an incident in Portugal, they sent in these service members to clean up this wreckage with no safety gear and they all died of cancer and the government denied it all. Also plutonium I believe.

Probably a lot