It's winter in Minnesota, I have a brand new roof, it's been below zero for a week or so, slight warm up. We have a leak on the interior bathroom door. Crawled into the attic and saw no sign of water damage, no water near the point of the leak. No other damage, just a slow dripping from above the door jamb.

Where does the fan vent? Outside or into attic?

Is it possible you have a water pipe running in area between door jamb and ceiling? If so you may have a connection that is not soldered correctly. You will have to open up one side of area above door to check this.

Chuck