Oumuamua was very curious and unusual object, if you were to probe a remote star system this seems like a really good and stealth way to do it. But we will never know if it was an outlier space rock with strange with strange path.
Cant prove it was alien so unusual rock is the most probable assumption.
There are suggestion’s that we could catch up to it in 26 years if we launch a probe.
That’s be an excellent way to dispel doubt
I'd say that the sea floor or dark side of the moon are good places to hide things. That doesn't say anything about motivation or probability though.
You could be interested in Erich von Däniken's books. Take them with a grain of salt, though.
Von Daniken's theories are transparently false. I read his books in the 1970s, and they were bad fiction even then.