What?...Are you saying that if we have been and are being visited by 'time' travellers, there is a 100% CERTAINTY that us 'civillians' ,or anyone else for that matter, would be aware of the fact.
Please note - 100% CERTAINTY.
Would they not find it in their best interest to be discrete? (To prevent being committed or experimented upon, apart from anything else?)
Would they not look just like us anyway?(I'm obviously referring to human time travellers).
You can come up with "yes buts", and "what ifs" all you like, but if there is no certainty (which there never is), that leaves the possibilty doesn't it.
If you can provide solid evidence that we would DEFINATELY know of their presence, due to their inability to avoid detection (recognition of themselves as a time traveller that is), I'd love to hear it.