Where happens to the poop ?

It basically dissolves into the water.

http://en.wikipedia.org/wiki/Turbidity

Turbidity is not something you can visually measure easily, especially at low levels. It's like smoke which contains individual particles that are each too small to be seen by the naked eye, but can be seen as the quantity aggregates and begins to interrupt visible light.

Your tank may "look" clear, but it's a relative term. At 100 NTU, water looks muddy. At 5 NTU it looks cloudy. But the NTU scale goes below 1 and we can't visibly see any differences below a certain level. So after a water and filter change, the water may soon look crystal clear with turbidity well below 1. Over time it may increase to 2 or 3, and under low lighting higher still before it's noticeably different.

Of course water that looks clear, isn't necessarily clean.

I like to change my floss filters when I see the dirt get half way down. I'm anal and I just imagine large chunks of dirt escaping the floss and spreading across the tank, and I just can't wait for it to get that far down.

A lot of the suspended particles end up trapped in the substrate, behind objects or generally in cul de sacs within the tank.