Originally Posted by JoeD
Yes. Water has to come from somewhere. Plumbing or roof leak is most common causes.
Joe's spot on. It has to come from somewhere. Most likely source is the plumbing vent that extends through the roof. They are usually a plastic/rubber flashing and will crack over time. Did you notice this after a rain? If so, caulk around the area where the rubber gasket seals to the vent pipe with a rubber based caulk. DO NOT USE SILICONE CAULK FOR THIS.
If this isn't the source of the leak look for water around plumbing fixtures supply lines, toilets, sinks, tub, any place that has a water source available may be the leak source.
Try this as a start to find the problem.