it gases me to say it, butt "potty talk" and "poop talk" rarely make it into a storyline, for good reason. it's just assumed that the characters' poop/pee are a natural occurance, and the writer doesn't have to display that for all to see. (i know Kalani peed, but that was a needed transition!)
when's the last time you read:
Michael: "Hey guys, wake up! We move out in 10."
Robbins: "Just let me take a quick dump first. I ate a bad MRE last night."
and then we hear it.

i was raised on poop talk with my family. sorry.

BACK TO THEORY CONTENT:
i don't think the Zeds smelling the way they do (like poo) is anything to speculate. they simply don't have the clarity to pull down their pants when their tummybox starts rumbling. Consider TOWTM didn't 'smell', as far as we know. they even assumed he was a normal.