Alien cover up

[Anomalocaris]

No, no credible evidence of alien having visited earth simply means it can not be asserted that aliens have in fact visited earth.   It does not mean it could therefore be asserted that aliens in fact did not visit earth.

By the same token, we should hold out the possibility that honeybees are fluent in Sanskrit - simply because there's no evidence that they speak it doesn't mean that they don't.  One might as well assert the possibilities of leprechauns, hobbits, sharks manufacturing bowler hats, or pandas composing long, involved discourses on the moral conflicts inherent in Dostoyevsky.

Whether or not aliens have ever visited our planet isn't a logic puzzle.  Physical phenomena require either physical evidence or compelling arguments, neither of which is present in the case of alien visitation.  When I ask, 'Do you believe that ETs have ever visited Earth?' and I get a yes, I'm immediately going to ask 'Why?'.  If the answer to that question is, 'Because there's no way to prove they haven't', I'm going to hit my respondent repeatedly over the head with a critical thinking textbook.

Boru

Er, no.  We have a good working knowledge, based or an enormous sample, readily further expandable to virtually any size we wish when we wish, of the amount of neurological complexity required to be capable of being fluent in Sanskrit.  We also have a good working knowledge of the basic requirements for any sound producing organ to allow speaking Sanskrit.  It is not that we didn't find basis for any in honey bee.   It is we found there are no basis for any in honeybees.  Therefore we might conclude there is multiple lines of profound and muturally support line of evidence of the absence of the ability to be fluent in Sanskrit in honey bees. Further, if some undetected flying critters out there do speak Sanskrit, we might conclude they must possess such a vast difference from honey been that they can't possibly be honey bees or closely replayed to honey bees. So we can say honey bees can't speak Sanskrit.

We have no such evidence of absence of alien visitation.   Only absence of evidence.  

Furthermore, We have no idea what sort of evidence a real visitation would leave. We have no idea how many potential visitors there are. We have no idea at what point in earth's 4.6 billion years they might have visited. So we are not in any sort of position whatsoever to pronounce whether some evidence we can detect ought to be there, or ought to have already been found and understood, if aliens had been here.

It's not only the absence of evidence, it is the absence of working knowledge of what evidence to expect.

As the trope goes, absence of evidence is not evidence of absence.