Can't there be a c) of "Who the hell knows, but there's a lot of evidence piling up." I'm with you in that I don't believe that an alien civilization that has overcome challenges of FTL, etc. would come to Earth to apparently joy-ride. There's something else that's causing trustworthy individuals to make these claims. At this point I just call it "magic" until a better word comes along.

One theory is that these bogeys have been here as long as we have and are basically automated tests, sort-of like the monolith in 2001 A Space Odyssey. It's ridiculous, but less ridiculous than a). The more I think about it, flying octopuses from Europa is less ridiculous than a).