Add common sense: even if an alien species was capable of it, it would be such an effort that they wouldn't come here for just for a picnic, there'd be a lot of evidence found of them.
This is actualy what the book "A Roadside Picnic" by Arkady and Boris Strugatsky, aliens visited earth for an unknown reason (scientists joked about them taking a roadside picnic on the way to some other planet) and where the aliens landed they would distort the fabric of reality, causing the laws of physics to no longer apply in some instaces.
The dead buried in these areas would come back to life, people who were in the area when the aliens were there would expierence weird thing like blindness when they left the area, and then there were the artifacts and anomalies created.
The artifacts were basically alien trash, but were wonderous things to humans, such as a rod that supplies infinite power. And the anoamalies could be random lackings of gravity in some areas, of areas that would litterally grind a man up.
From what I can gather from that book, aliens have most likely not been to earth.