Well it could be like a civilization that has an airplane, that might stop somewhere to watch a primitive tribe and then move on. Or maybe like in Star Trek, they might observe but interfere as little as possible so they don't negatively affect our development. But as for the cases of Betty & Barmey Hill, and Travis Walton, their stories have not been debunked as far as I know. So what do you suppose happened? I was reading something about Travis Walton one time, and it said during an interview (I think), he said "it's different when you know for sure" (regarding whether extraterrestrial aliens exist). I've also heard a theory. Incidents of UFO sightings and such really increased after world war 2 when the US dropped atomic bombs on Japan. I've heard a theory that extraterrestrials may have noticed and it makes them nervous, and they want our leaders to stop. Maybe a tinfoil hat thing though?