Drain and sewer cleaner here. In a situation like this my company would perform a smoke test. Access the sewer line and pump smoke in which would reveal where it can escape. Sometimes it's a tiny gap in pvc glue at a hub or a tiny crack in cast. Sometimes there's no smoke and instead a mouse died in the walls. Chasing smells is tough without smoke. Another reason can be if the fixtures are not used frequently the traps dry out and smell quite bad. Running hot water to see if the smell is more noticable can help isolate it to sewer lines as well.