may be [hopefully] civilizations reach mental maturity preventing such pollution of their galactic neighborhood with self-replicating trash before being able to perform such pollution. "Morris worm" at galactic scale :)

With regards to aliens - i see the Bible as exactly historical document of such an unplanned/unauthorized contact by an alien observer gone rogue. Contains all elements, like medical nanobots, [food]replication, specific lessons that violence is the main obstacle of civilization development, blue light sterilizing the cave with decontamination workers in white overalls and some devices on their backs (angels :), etc... Hunt for the lost sample of the alien's DNA - Holy Grail- which also contains those nanobots.