As I said along time ago in a similar thread, I don't think they are aliens, they are UFO's though in the strict definition of the term, Unidentifiable. I would say we will slowly find out that it is some governement of private entity that is testing their vehicles.
Aliens that can traverse the massive distances of space would be so much more advance than us it would be like us trying to relate with earthworms, I don't know why no one has put forward the idea that we maybe the most technically advance beings in the universe, it is as plausible as beings travelling 1000light years to say hello to a species that is so inferior to them that they probably wouldn't waste their time on us for very long before they had all the info they needed.
So far they have been supposedly visiting us for over 30 years, this doesn't seem logical, they could get all the information they need in a study lasting a maximum of a year considering our compared simplicity to theirs.
Aliens may visit earth for extended periods for other reasons though, think of an Alien race, that has been around longer than this universe, it is so advanced that it can blend into our society its abilities could be godlike... it may just come here for a holiday :P
I know I would.