Jay - thanks very much for your insight. Unfortunately, it's not a gas leak as I just replaced the toilet and have a pretty good seal. I'm wondering if it soaked in somewhere behind the baseboards. Since we just ripped those out, do you think I should let it air out for a week - in case it's still damp and mildew-y and then try spraying some heavy-duty lysol or something behind in case that's where it's coming from? Thanks again!