There are pretty low chances of an alien species more advanced than us existing in our galaxy yet alone in our solar system. If they've purposely isolated us and are hidden, then well that's another topic suitable for Fermi's Paradox. Micro-organisms are more likely than a highly developed Alien race but the chances of even those micro-organisms are low. If we look outside of our solar system and galaxy, then anything's possible really.
For all we know, there may be Thomas the Tank Engines flying around in some place in the universe. No proof of Alien existence, yet high probability of it's existence...