1. If aliens would have been in contact with humans it would come out fast enough, such a thing can not be kept a secret.
It depends on how they are. If they are all controlled by one leader or a hive, they could very well be evil. Maybe it's like X-Files or Dark Skies?
They simply want our planet, and use us as slaves for their bidding. Adding to the scenario, maybe they aren't many in numbers, therefore they prefer
to take our planet without us even knowing it... until it's too late.
And if they have travelled so long, they probably have tech that surpasses our tech, they can probably cloak their ships if they wanted to stay out of
sight.
2. If aliens fly in and out of our atmosphere all the time, you would be able to see it with the naked eye, not even mentioning
radars.
They fly rather fast. Maybe they found a way through without having to enter the planet as capsules from spaceships have to. Besides, the UFO's are
often seen in forrests and deserted places, so they would probably use those areas as an "entry".
3. If aliens did have more advanced technology then us, they surely wouldn't bother traveling that X million miles to show us a glimpse of
their spaceships before they go back.
How do we know it's not just probes? Send it here, collect info, and then self destruct (Jupiter, Saturn, Uranus and Neptune as well as the sun
itself are good "ovens", none of them have solid grounds)...