Pee trap

Certainly could be what you are smelling hehe...
PEE or POOP...

I use to get bad smell by leaving my ceiling vent in the bath room opened... Pulling from the POT I guess...

Roy Ken

I gotta believe there is a trap on your shower drain somewhere. Can you narrow down the area that its coming from. Open a sink cabinet and see if the smell is worse in one of them. My guess is an air admittance valve.
Clicky

Have you checked the air admittance valves near the sinks? Sometimes those P traps need to be cleaned out. Take 'em apart and you'll be surprised how much gunk is built up.

LOL.. pee trap. Interesting take on P trap.

yes, but may depend also. I think my last TT did not have a trap in the bath tub. It went straight into the tank.