Bathroom air vent drip
There's a few possibilities, and it's hard to pick one given the area 


If the inside of your house is warm and moist, and the attic space or outside of your house is cold, the water could be condensing inside the pipe. 
There could be a plumbing leak in the space above the fan that's leaking out the fan as a path of least resistance. 
You could have some other vapor barrier issue in the area around your fan that's causing condensation.