I've been in an extremely similar situation.....and might even nail this one on the head for you.....water pipes. In an old house we lived in, the water pipes ran under the floor......at one spot in the hallway, there was a smell, and it was not a smell unlike feces...as you've said.....it took us some time to figure it out...but it was indeed the carpet.....and the reason it was the carpet was "what lies beneath". In the floor, in that very spot, was a pipe joint...ad it was leaking...it only leaked when there was a good amount of pressure on it...a shower wouldnt do it..the dishwasher or washing mashine wouldnt do it....but a combinaiton of more than one WOULD.....the water meter was at one end of the house, the appliances requring water were at the other...and the explanation for the joint that WE got...was that "they don't use one straight piece of pipe in older houses, because there was no "Support for the long piece....." Well, the padding on the carpet was almost two inches thick.....so it was the padding that was soaking it up when it would leak through the wood floor (it leaked similar to a garden hose with a hole in it...). We never knew it because we never really "felt" it.......wetness...and the smell would go away once the mildew dried out.....until the time came when the off chance hit that ALL stars aligned and put that much pressure on the pipe for it to leak again. Hell...we tore up the carpet and everything else trying to find it...and when we did...we felt quite ashamed we didnt just go to the basement and check it....hey..maybe thats it....maybe it's not...but it's worth looking at...
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