Hi a few weeks ago I noticed what I thought to be an old poopie smell in the bathroom, in the morning, before anyone was even in there. A friend said it didn't smell like a dead animal, and related it to a possible sewer vent problem, after checking the pipes for leakage below the bathroom, in our crawl space. (he said because he is a builder he has smelled that smell before on roofs by sewer vents) The smell is clearly coming from the wall that backs up to outside, and also can be smelled by the floor in that area ( when you smell the toilet or the sink, it doesn't smell at all). A plumbler came in and said he didn't think the smell was a sewer vent, he said he tested it by flushing the toilet and filling the sink up with water. I have cleaned the wall and floor several times since, it has been a few weeks and the smell, now smells like possible urine, or like, excuse my presentation of this, but for lack of better words, like someone who is dirty, and has just gone to the bathroom pee pee. This smell occurs, first thing in the morning and persists all day, without regard to flushing toilets, running water, heat, etc. Here's the other situation, wev'e had a cat problem in our yard, and a friend of mine who has had cats said that outside, right by where I am having the problem inside, she said smelled like cat urine, now, I totally can't smell it myself. But maybe they are peeing in that area outside, and the smell is seeping into my house. I heard if you put mothflakes out they will not come around, and my neighbor also has agreed to stop feeding the cats. I put the mothflakes all over that area a few days ago. It still smells. I am a very clean person, and none other of my bathrooms have ever had this problem. Also if you smell where the heat is coming up , the smell is not coming up through there, it seems to be coming through the wall and the floor.

Look down the drain at all the traps. If they're full of water then you should be OK, but if there's almost no water in any of the traps then you could have a vent that's blocked. Also you've tried everything else you might want to consider replacing the wax rings in your toilets. Sewer gas can escape a defective ring yet not leak water when flushed. The wax ring can emit sewer gases from the base of your toilets and you will swear it's coming from the shower/tub drain, lavatory drain etc.
Sewer gases have a way of dulling the olfactory nerve and make detection nasally impossible.
You must realize that smells and weird noises are the hardest to track down when you're on site and just about impossible when you're not.
All I can do is point out likely spots. Good luck, Tom

Are you in a home with a crawl space? Do animals have access to the crawl space?

Yes we have a crawl space, and I don't think that animals have access , I think the cats are peeing next to my house alongside where the bathroom is. The floor of the bathroom smells also. I went away for 2 days and it still smells!

I am not a catlover and I don't want to make anyone angry so I will let others handle this one.

It is probably the cats outside. I would bet that when it is very low humidity (less than 50%) outside there is no odor. When it is above 50% and the furnace is running, you get the smell Assuming that the furnace is inside your living space and not in the crawl space.

If this seems to be the case, you can eliminate the odor outside by spraying the area with odorxit concentrate mixed with lots of water. It is the only thing I use for pet odor problems.