I've seen some weird lights in the sky before--a plane-looking light which flew around in no real pattern, slowing down and speeding up randomly, but I'm not convinced what I saw was alien. But I'm sure that other people who saw the same thing (like some family I was with, who were more open to idea that it might be alien when I showed them it) might be more convinced then I was. That right there is why the "ancient aliens" hypothesis as argued by Däniken is so compelling to many people. Anything which gives a convincing answer to mysterious things (like Chariots of the Gods points out) will make sense to many people who haven't researched the subject more or otherwise have some level of skepticism.
Sagan said it would take a billion.
I don't know why Sagan would have said that. The Milky Way is at most 180,000 light years in diameter, with the majority of stars in a region 100,000 light years in diameter. Let's say the average colony ship can travel at 10% the speed of light, which is perfectly reasonable considering hypothetical propulsion methods which don't violate physical laws. The most distant stars in our galaxy can thus be reached in not even 2 million years. If the human population experiences exponential growth, doubling once per millennia (perhaps a reasonable growth rate for a post-scarcity society which might colonise the galaxy, not even considering the demographic development of space colonies!), then the human population will easily be enough to have a fully populated Dyson swarm of 1 quintillion people for all 100-400 billion stars in the galaxy plus every single brown dwarf and rogue planet. Even if the
biological population remains low (a possibility, and maybe the more likely one considering demographic trends), then the
digital population is likely to expand at exponential rates as new AI beings or digitised human beings are born or created for whatever reason, and these beings could presumably "incarnate" themselves in physical bodies if they so chose to. Like biological beings, they'd seek out new sources of energy (stars, brown dwarfs, etc.) to sustain themselves.
This means the galaxy can (perhaps
will is the best word) be filled in no more than 2 million years, and possibly no more than a million. Since aliens can and would do the same to their galaxies if they could, we'd expect to see this. The fact they haven't means they either do not exist, are too primitive to do so, or we cannot see them because of light lag.
Then why are they usually associated with flying craft that go up?
Because even though you can travel between dimensions (meaning you don't have much reason to expand outside your solar system, or even within your solar system), there still might be some reason why you would want to fly out of an atmosphere without using your dimensional drive or whatever (assuming that some accounts of UFOs aren't just that--some aliens using their inter-universe drive). The theory that aliens have some way to travel from another universe to ours seems compelling, because it explains why we don't see Type II or Type III civs. If FTL travel were to be possible, then the odds are high that it would be via wormholes which according to some theories don't link between points in this universe but points between universes.
I don't think anyone detected a meteorite before it entered the atmosphere. Superluminal travel--if possible--might also explain it.
We've detected several impact events before they hit Earth. See
2008 TC3,
2014 AA, and
2018 LA last month. Granted, none were discovered more than a day before the impact.