The only reason I found it was because a bird dropping had splattered just on the edge of the frame. The sponge touched the mesh and turned instantly pure black. That was what made me investigate further ...
I actually discovered this on my car on Saturday too.
I gave the inside a vacuum.. including the wind deflector... wiping the interior with my yellow cosco microfiber and noticed it got black with something... traced it back to the wind deflector.... so had to literally wash the cloth.