#3 - they/we fled Earth (due to some catastrophe?) and have been living elsewhere in the solar system? Perhaps in hollowed out planetoids? Visiting the recovering planet and Waiting for the right time/conditions to return to stay?
Perhapse they are the Nephilium ie. Giants as described in the Bible, not giants in actual size, but Giants in the same way that Power in terms of Technology is described “Technological Giants” per se....
Could be they left due to the ice ages or end of the ice ages.....?
Dunno, but #3 seems to be the most plausible of the group. No t saying I believe it. Just that IF the explanation was one of the four then it is the strongest candidate.
One comment though about interstellar travel: if we are talking about going faster than light or “bending space/time” then it is impossible to conceive of how it could be accomplished.
Slower interstellar travel is however almost with our grasp even now. At a speed of 1/10th the speed of light (admittedly very VERY fast)a trip to a star 63 light years away could be made in about 630 years.
All you need is a self contained planetisimal in which generation after generation of spacefarers will be born and die until the craft reaches any promising stars, where it parks in orbit (maybe like that of 3753 Cruithne ) and sends out smaller craft for exploration.