If you have the same amount of water in the closet that you have in the bathroom, somthing is wrong! It may not be related but, still...
I am thinking that there may be a vent pipe, (stink pipe), in the wall behind the toilet that may have leaked from the roof or generated condensation at some time.
I think I would cut an access/inspection hole in the back of the closet, near the floor.
This is where I would look for signs of mold and or mildew.
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