Make sure all traps have water in them.

The post does not say where you live so I cannot tell which plumbing code you are under, however this house is new to you and if you are under the IRC (International Residential code) or the IPC (International Plumbing Code) there is a very strong likelyhood that you may have an "AAV" (air admittance valve), AKA "Studor Vent" or "Cheater Vent" instead of a true vent pipe in your bathroom. One quick way to get an good idea if you have an AAV is open the cabinet under the sink when you smell the odor. If you have a defective AAV the odor should be much stronger under the sink.

AAV's have a spring loaded plunger that will allow air to enter the drain line to prevent a negative pressure that would suck the water out of your traps, but, when working properly they are closed to prevent sewer gasses from escaping from the pipe.

Given that it is a mechanical device, it also stands that it is subject to malfunctioning.

AAV's must be installed at a location where they can be periodically inspected and allow access for repair. Typically for a bathroom they are installed on the lavatory bowl waste arm in the cabinet under the sink.

Look at the sink drain line. If you have an AAV you will see a Tee fitting on the horizontal line coming out of the wall and before it connects to the P-trap from the sink bowl. You will then see a vertical line attached to that tee which rises a minimum of 6" above the horizontal line and generally it will rise up behind the sink bowl. You will then see a Cap on the top of the line, but if you look close you will see either some slots or a square opening on that cap fitting. That cap fitting is the AAV.

AAV's are mounted by means of ordinary pipe threads. On the top of the vertical riser there is a female thread adapter and the AAV is simply screwed in. To replace it you simply unscrew the existing AAV, than apply teflon tape to the threads of a new AAV and screw it back in place.

You can get the AAV's at any hardware store or home supply center. Personally I prefer to go to a True Value or ACE hardware and you will be more likely to find a clerk that really knows what their talking about to help you.

Apart from the wallpaper, are there any other porous materials in the vicinity of the toilet? Standing urination tends to spatter the surroundings as it hits hard porcelain.

The smell may not be plumbing related. Certain plywoods will give off a disagreeable odor under elevated temperature/humidity conditions. These could be in cabinetry, shelving or subfloor. On the latter, can you inspect the wax seal area under the toilet via lower floor or crawlspace?

We live in Westchester county in NY. The home is from 1885, and I think the bathroom is from 1993-1998. I should also note that there is only a window in the bathroom and no exhaust fan.

Just checked and there is no aav that I can see. It looks like just regular fittings to the sink. It is a very basic bathroom.

Years ago I moved into a house that formerly had a family with three young boys. Basically, they pissed all over everything in the process of learning how to do it right. The structural wood in the floor all around the toilet was dry and solid but well-saturated. Whenever the humidity went up -- like even after showering -- the odor bloomed. Only solution was to replace that structural flooring material. The joists were not affected. Problem was solved.

To go along with what asolo said, I have a friend who bought a house where an elderly gentleman lived and he had a habit of peeing on walls and floors in the bathroom and bedroom, causing the same situation with odor that you are having. They ended up having to replace drywall, flooring and floor joists in the affected areas to get rid of the odor.

I'd first do exactly as I would if trying to find a pet urine issue. Get a black light from the pet store (pretty cheap) and look for the glow where urine has spattered. Try soaking those areas and the area in the vicinity of the toilet in a enzyme containing product designed for animal urine stains like Nature's Miracle. If the issue is due to overspray soaking into the floors and walls, then the only solution that may eliminate the smell is the replacement of those floors and walls. But, I'd try the enzyme cleaner first, and then maybe a coat of a oil based primer followed by a coat of paint. If the grout is affected, then actually sawing out the grout and redoing it would need to be done. With one of the new multi-tools on the market and a grout removal blade, the chore isn't as difficult as it once was. However, it's difficult enough that you might choose to replace the tile entirely rather than replace the grout.

A client of mine had a similar mystery odor. They exhausted almost every possibility...clearing vents, replacing toilet wax ring, floor cleaning, drain cleaning, etc.

Finally we found out that it was coming from behind the tile when it got wet during a shower, both in the corner and around the trim plate. While the grout appeared intact, some water was getting behind it and activating the smell. I even covered various parts of the bathroom with plastic sheeting to help isolate the smell, sniffing underneath each after some time had passed.

Hope that helps.

There was just an article AOL posted on this issue. They claim most odors come from the toilet itself. They suggest dumping 3 cups of white vinegar into the toilet. Just passing the info on, you may as well give it a try, who knows, it might help. I wonder if you scrubbed the whole bathroom with the stuff, if that would help.