What if aliens were here and we just didn't recognize them. I am not suggesting that they blend in or are 2-legged creatures, rather that they are creatures that we think of as terrestrial but they really aren't.
For example the COELECANTH. It was presumed extinct and then suddenly started to reappear. What if they disappeared because they went back to their home planet? They could have spaceships or other transport and we would never know.
There are plenty of other possibilities too.
I mean, think about it. If apes and dolphins have been around for virtually the same time periods and we claim apes became advanced and evolved into humans, what about dolphins. Are they not even at the tools stage? What if they had advanced technologies that we couldn't even understand??
Maybe this is just too wild, but then again so was the theory of the earth being round and revolving around the sun.