Lets suppose for a moment that an amateur astronomer did happen to spot an "alien spacecraft" in orbit while imaging something else. The question is, would he or she realize the extra-terrestrial nature of what they happened to catch? If it's just sitting in orbit it'll move like any other satellite, and provided it's not excessively large and/or in a very low orbit, and the telescope/CCD combination doesn't have a high arcsecond/pixel resolution (most amateur astrophotography is gravitating to wide field work anyway), the alien visitor will look just like any other satellite; an annoying streak in the image. Now how many amateurs bother to check and see which satellite showed up in their image every time that happens? And even if they did so and returned no hits, they'd assume it was just a classified spy satellite whose orbital elements aren't easily accessible. In my opinion, if an amateur were to accidently capture an image of a "visitor" in orbit they'd never even realize it. And if an advanced race were to visit I doubt it'd be for anything that required a lengthy stay; any sightings would be exceedingly rare, if they occured at all, and would ultimately be discared in the pile of hoaxes, lies, and mistaken sightings. It's like an extremely bad signal/noise ratio. We'll never know if we've been visited or not, and though we can say the odds aren't good, there's too much nonsense out there to find out if even one sighting was for real (and even if it was it'd never be provable scientifically). I know my story is just that, a story. It's not supposed to prove anything, but as a personal family experience it's shaped how I view the subject of hypothetical alien visitation. I don't rule out the possibility, only the "provability." You couldn't prove an encounter happened unless they went out of their way to make it provable, in which case the whole world would know about it. There'd be no chance for a "government coverup."