Here's my problem with the Zoo Hypothesis:

You know how hard it is for us to find them in all that immense, inconceivable vastness of space? It's probably just about as hard for them to find us.

Space is not indexed. It's not made of octrees. To find something, you have to get within radio signaling range in 3D space, of which there is more than any being smaller than a solar system cannot realistically comprehend. If anything else intelligent out there is also bound to the same dimensions as us, and exists at a scale where they inhabit a planet of any type, there's pretty much no way we should ever know about each other unless we already get each others' radio pollution.