While technically true, this is not a reason to believe that aliens have visited this planet. Again - you
cannot logically use "we don't know" as a reason for knowing! It's absurd!
And as for "the entire body of sightings from the dawn of mankind up to the present day", I invite you to actually examine them. The earliest consist of imagery that we don't necessarily know how to interpret without intimate knowledge of the cultures that produced them. The next most recent are
actually descriptions of angels, devils, demons and witches that have been
reinterpreted in modern times to refer to aliens. The most recent are so obviously influenced by modern pop culture as to be clearly nothing more than a a sociological phenomenon.
All in all, the evidence "from the dawn of mankind" is
extremely poor in ever conceivable way.
Think about this. If you were able to build a spacecraft and travel to other worlds, would you:
a) land in their biggest city and say "take me to your leader"?
b) land out in the country beside some guy who no-one's going to believe and strut up and down wearing silly antennae on your head making "beep beep" noises?
If I traveled to a planet that earth was much more technologically than, I sure as hell wouldn't land in a big city and say take me to your leader. You might get killed or kept in a zoo. I would discretely observe to get a lay of the land first. From those observations one might conclude that it wouldn't be wise to interact with them. If so, I sure as hell wouldn't just fly away, I would want to isolate some and study them; advancement of science and all that.
The body of evidence that supports alien visitation is greatly exaggerated. A majority, if not a totality, of "sightings" have a rational explanation - even if we do not currently know what that explanation is - that does not involve aliens visiting our little planet.