Picking up the toilet to check is the least expensive thing to try forst. A lew wax ring os only a couple bucks.

If no problem is found(and a leak there indicates improper installation), and there is a history of water problems in that wall, that is pretty much the only other place.

Thanks handymac. I was wondering if there is any sort of tool that detects moisure, too, so I can narrow down the area of wall to open up? (if it is not the toilet)

I just noticed I hit all the improper keys in my first post(Insert embarassed icon here).

I do not know of a meter like that. I would start where the moisture showed a problem---window area. From that point, just taking small pieces out until there is no more evidence.

Got to share this, in case anyone else experiences this problem! We have had an ever-increasing mildew odor coming from a bedroom/bath area. There was NO sign of moisture to be found. I paid to have our septic tank pumped, our air vents cleaned, our air-conditioner serviced, and the wax ring on the toilet replaced. We went into the attic and checked the vent pipes, and cleared around the bathroom exhaust vents. Nothing worked. I was about to tear into the walls and decided to check the bathrub drain. BINGO! A huge plug of hair had developed and was the culprit. I can't believe how simple of a fix it was, or how terrible it smelled. Sweet relief ...