What is the cause? the spot is not wet just a stain.
If there's a stain from water then there is water. But don't immediately replace your roof. (Unless it is due. Chances are it may be poor ventilation or a nearby vent leaking or a combination of both. I would bet that is an exterior wall. I would also bet you are having frost buildup in your attic which melts with warmer weather but never really creates enough moisture to actually drip.
Check your attic ventilation. If you look up there, you should be able to possibly see daylight around the perimeter or there should be ventilation chutes placed about 6-8' apart.
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