I don't know if extra-terrestrials have ever visited Earth or not, but I do have a few views on the subject that I think are interesting to think about. For example, consider the scope of the universe. Few of us have the mental capability to realize just how big our universe actually is. I have a model of just our solar system, the sun, the Earth, and the other planets, called the "1000 Yard Solar System". When you set this model up, our solar system stretches over 1000 yards. The sun is the size of a soccer ball, Mercury is the size of a pinhead, Venus and Earth are peppercorn sized, and the distance from the Earth to our moon is 2.25 inches. 2.25 inches!!! That represents how far man has been off the planet. Can you imagine trying to go to one of the planets like Uranus or Neptune with our current space technology? It would take decades just to make the trip!
So if I were God, and I had this really big room to set up all these planets, galaxies, and solar systems in, that were separated by this huge expanse of space, why would I stop with just one planet with one type of species called humans? Maybe their are other species with advanced scientific advances that dwarf ours, and they are much too sophisticated for us to detect them, much less go visit them. It certainly provides food for thought.