Heretical view: intelligent alien life is everywhere swimming around us. We are just not capable of really understanding it. When a human walks past an ant, maybe the ant senses the shadow and the blockage of the sun.. but it isn't equipped to recognize the human. Likewise, I suspect we are in a similar situation where even our smartest minds and best scientifically advanced sensors are simply incapable of properly detecting these forms. Remember, the average age difference between planets in our galaxy is roughly 2 billion years... just imagine that an alien life 2 billion years older than humans have found easier ways to travel than little metal flying saucers. The question then may be if it's ever possible for us to sense these higher (plausibly extra dimensional) creatures. I suspect this will be a more fruitful direction to investigate rather than assuming we just happen to be the only exotic intelligent life in the galaxy.