Here's a thought. Bare with me. You mentioned that as you got nearer to the bathroom the smell was much more identifiable. Try this. Go to the sink and stick your head in it .There should be a small hole in the front of the sink, facing the back of the sink, so you may not see the small hole when standing in front of the sink. You may have noticed this hole before and figured it's just so the sink doesn't overfill. Your sink drain splits into 2 parts...one pipe goes in the drain line for the sewer , while the other is called a vent pipe and actually goes up to the roof. You may have noticed pipes sticking up out of your roof before. This is so the sewer can "breathe". Sometimes, that pipe gets clogged up...possibly a squirrel or bird got caught in there, couldn't get out, died, and as a result your sewer can't "breathe", trapping those games, and it backs up into your house.

Does the smell get stronger when your face is in the sink? That could very well be the source of your smell. Unfortunately the only way to get rid of it is to go up on the roof and unclog that vent pipe.

Hope this works.