Number four was the theme of a recent Dr Who double episode.
There are artifact objects in our solar system which have not been put where they are by our current civilization. People poo poo that reality but I happen to know a person who worked for NASA airbrushing out the objects when photographed during NASA missions. That lends credence to your number three posit.
I suspect those objects remain as evidence that a previous Earth sourced civilization got off the planet in some distant past epoch.
I cannot reason out why an extra-solar civilization would leave debris behind, having gone tot he great effort to fabricate their materials. But if a civilization sourced in Earth's past became suddenly destroyed then the remains of that civilization could survive as artifacts on the Moon, on Phobos, and on other solar objects.
I don’t see why the government would have such an interest in airbryushing out such exciting news.....
Oh wait, CONTROL is why...
IF we found out that an ancient race of humans ventured off this planet and is currently living in say the asteroid belt, how would we respond as an american people?
I think it would encourage us to spread out into the solar system and would weaken the power of the central authority, which is something the government would not like at all.
We would also want to meet them on equal footing if they were currently living in our solar system and if we did and a group formed an alliance with them this would also weaken the central control the central planner have over us.
That is probably why they would insist on airbrushing the past so to speak, so that it is like the pictures that the KGB has airbrushed....