To work properly if there is a vent, it would need to run up above the flood rim of those drains connected above it or have its own run to the roof. There could be one, but it is not installed properly and the trap is getting siphoned dry in certain situations. Even if some traps are vented properly, they may not be vented within the proper distances which can also create problems. Any way you can tell how it is installed, or would you have to tear out walls?
Some installers take a shortcut when building a shower on a cement slab and either omit the liner, or don't install it on a presloped floor. This allows moisture to filter through the grout or cracks and either continue into the slab floor or pool in the flat liner. After awhile, that trapped water can start to smell like a swamp, which could also account for some smells.