So there's water in all the traps, you have no visible leaks and the pipes are free of clogs. What next?
It seems natural to assume that the toilet would be the obvious source of any sewage smells, but if there's water in the bowl, chances are the commode is not the problem. (The water here acts as a smell barrier -- just like the water in a sink's P-trap.) But if the bowl doesn't fill up as it should, the problem could be a broken seal, in which case it may be time to call the plumber.
Things get even trickier when the smell is coming from inside the bathroom but the problem itself is somewhere else. A roof vent blocked by a bird's nest, leaves, or snow and ice prevents fresh air from coming into the plumbing system and gives sewage gases nowhere to go but your bathroom drains.
A vent pipe that's been improperly installed or cut can also send offensive gases into your home. (Believe it or not, contractors have been known to vent these pipes into attics and crawl spaces instead of out through the roof.) And a cracked vent pipe can be even harder to track down, since the break is probably hidden somewhere within your walls. A plumber can help you locate a vent pipe leak with a device called a smoke machine, which fills the drain system with a harmless visible smoke. When the smoke finds its way out, you've found the source of the leak.
If you want to know even more about what's causing that sewage smell in your bathroom (and who wouldn't, really?), check out the links on the next page.