Trolling is naughty... why don't you tell us what you really think?
Personally I doubt any real encounter has occurred... simply because people would not be able to keep that sort of secret for long.
Of course having said that if an alien race was able to get here undetected it would not be a huge stretch to think they might remain undetected.
I remember speculation that the standard alien... you know, the type they call the Grays or whatever, are actually future humans coming back in time and that this is what we will evolve into, but that assumes we don't kill ourselves first, which I am doubting still.
I think we will likely invent a nano technology that was designed to do something banal like kill germs... some super high tech mixture of robot and animal genetics that finds germs and kills them that somehow mutates and starts killing all living organisms it finds or something silly like that... in 1,000,000 years when all the crap we made is gone and the earth has cleaned up things again those engineered bots that wiped everything else out will start a new range of cybernetic organisms, part machine, part biology, able to repair and maintain itself and build new populations...