My home is built on a slab as are most here in TX. My hall bathroom (sink, toilet, tub/shower) develops a foul odor like an open plumbing vent after about a week of non-use. I suspect a leak or evaporation in the trap but I can't get to it because it's burried in concrete. For now, when I notice the foul odor, I flush the toilet a few times and run water in the sink and tub. I also use "stick-ups" to cover up the stink.
I'd like to know what might be the real problem and what I can do about it.