The scariest thing for humans would be to find that they aren't "aliens", but a species that took the technological path of survival long before humans did (and maybe several of them), that they have underground bases on the moon, Mars, and other places in the Solar System (possibly even in the larger asteroids), and these species don't actually give two fucks about what humans think of themselves. And that humans are being currently sussed out by their wildlife managers for ... management plans.
The scariest thing for humans would be to find that they aren't "aliens", but a species that took the technological path of survival long before humans did (and maybe several of them), that they have underground bases on the moon, Mars, and other places in the Solar System (possibly even in the larger asteroids), and these species don't actually give two fucks about what humans think of themselves. And that humans are being currently sussed out by their wildlife managers for ... management plans.