[The Second Law]

A few thousand years is an awfully small amount of time to have noted a visit, especially considering how it would take manymany years for any sort of signal to be sent to Earth and back, and even longer for someone else to get here, and considering how we've only had a few decades to even consider sending a message into space or receiving one of theirs here.

Basically, we could have been visited already, but before humans were populous. The amount of time that we've had the capability to note these things or even facilitate them is far too small to consider a visit from intelligent life that exists elsewhere in the universe.

It's no mystery to me.

--Second