Our Bathroom Smells Horrible, But I Can't Find The Source?

In the past couple of weeks our only full bathroom has developed this horrific oder and I cannot figure out where its coming from for the life of me. We keep a very clean home. My husband is a complete neat freak (Its annoyingly adorable. lol) He bleaches the tub and the toilet once a week and uses Drain-o in the sink and tub almost just as often. We also mop the floor and wash all of the linens, including the rug about twice a week. The trash also goes out quite frequently. I even checked behind the toilet and under the sink to see if a mouse died or something. It is seriously that bad. I won't use the toilet in there anymore. Any ideas about where it might be coming from? I'm totally stumped.