If this was my job? We would remove a piece of the drywall to see what is happenning behind the wall assembly. Guessing what is goin in is not a good solution with water issues. If there is water behind the sheetrock, and I suspect a little mold, then you know if there is an issue with the exterior wall assembly. Sheetrock is the easier thing to replace, and if there is damage, you want to address it now before winter.
Hope this helps, and use a hose to get the wall wet.