I've heard people describe musty odors in their homes, and urine-like smells, but the one I have is different. I'm a new tenant in a house that was built in 1910. Big farmhouse type home. The water pipes are on the far end of the apartment (back side of the house), where the kitchen is (left) and bathroom is (right). The smell comes from the bathroom. I noticed it when I moved in 3 months ago, have cleaned multiple times since, and it's still there. Sometimes very faint, sometimes overwhelming, and permeating down to hall toward my bedroom. In the bathroom closet (beside toilet) is a hole on the bottom, and a bunch of pipes & stuff that lead down to shear darkness, (some other part of house?). I think the odor starts there. It's a warm smell, very heavy and lingering, and the first thing I think is OOOLD, greasy, chicken fat. I used to think it was my landlord cooking dinner, but hubby says I'm wrong; her kitchen isn't even beneath my bathroom! (duh!)
Hubby says it's a hot-water-pipe smell (though I've smelled that before, & this smells different.) What could this be? I smell it every single day, and nobody's bothered by it but me. It does NOT smell like sewage. OOOLD, greasy, chicken fat. Exactly. The times I smell it are random, (though usually in early evening into later evening,) and sometimes it's vague, and other times it's horrible.

Try stuffing up the hole in the floor first with some kind of sealant and see if that helps.

Thank you for your response; I could do that, and that is the obvious course of action if I wish to just make it stop (which in the long run, I do... ) but I'm more interested to know what the heck the smell is. I've never smelled anything like it before, and it's such a mystery. When I smell it, it drives me crazy that I can't place it! Once I know what's causing it, I might be satisfied to just plug it up. But what if it's some pipe backing up? What if it's sewage of some sort, or waste, (even if it doesn't smell that way) that needs to be dealt with before plugging?