This watermark has appeared near my kitchen vent's exterior. It is between the kitchen and the master bedroom on the second floor. However, there is no water pipe around the area of the water mark, and, no air leak from the kitchen vent pipe.
Does anyone know why this watermark could have appeared in this area even when there is no water leak?
I think I have found the cause. There is a large gap around kitchen vent's pipe which caused air leakage during the winter time. I heard from multiple sources say that it could lead to condensation inside wall. I've filled it with gap filler and hopefully it could solve my problem.
Two pictures attached. One is taken before fix and another one is taken after fix.
Thank you Collin and Rodney.
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