Okay, in fairness, you might have imagined them like this, but it could be that alien intelligence exists in a non-biological form.
Whether intelligent or not, a self replicating system could conceivably propagate itself throughout the galaxy without the continued input of a biological being. Robotics and AI are obviously unlikely to arise spontaneously, but once switched on, they could act as a completely self-sustaining system exactly like humans.
There are those here on earth who think the idea of self-replicating artificial life is not just possible, but inevitable. There are even those who think that it could be the dominant life form in the universe.
This throws a spanner in the works for our attempts at detecting alien life via the medium of biomarkers. If there's a civilisation of artificial minds that are living in the form of digital uploads in a supercomputer, there's not much chance that they'll be kicking out too much methane for us to detect.