If aliens had the technology to cross space and time to bother us, why would they?
It makes no sense. They'd have the ability to monitor us from great distances if they really wanted to know about us. They wouldn't zip around in ships. *WE* are just now getting into drones. Imagine their drones. In fact, chances are that they'd "go small". I'll explain.
The human race's defenses can all be defeated with scale.
Have you seen how small we can make shit? They'd have the ability to make things thousands of times smaller than we can. Specs of dust powered by zero-point energy that can communicate faster than light while gathering full spectrum data. Robots that hide out on strands of DNA. They'd have stuff like that. Harbor no illusions that we'd even have the ability to defend ourselves, let alone detect them.
The problem with imagining aliens is we are limited by what we can imagine. That's why we used to think "they" were from the moon, or mars. That's why if "they" really were here we'd never know it. Now, let me really fry your brain:
Consider that in 100 years time we've gone from imagining aliens as actually showing up in vehicles to you now realizing that they'd never bother. They'd be smarter than that. Our understanding of a would-be visitor evolved because of discoveries we've made. Now imagine how much more advanced they'd seem in 100 more years when we figure out more about how the Universe works, and we finally grow up and stop believing in fairy tales.
Seriously. There are no aliens. Let it go.