Incidentally, scientists are always saying it's impossible for aliens to contact us because of the thousands of years it would take a radio signal to reach us, and thousands of years for their spacecraft to reach us because nothing can travel faster than light.
But what if they're trying to contact us telepathically, and what if thought waves travel instantaneously ?
For example a group of aliens on the far side of the universe might sit down and concentrate on sending a "thought message" out into space saying something like "We are on another planet, can anyone pick up our thoughts?"
Let's suppose some people on earth CAN pick up that thought message, but the problem is they can't interpret it correctly because it originated in a totally alien brain, so the human mind sort of freaks out and hallucinates, and the person thinks he sees UFO's and aliens.
It's just a theory but would explain many so-called close encounters.