For one thing, it could be condensation from the pipes. As the pipes deliver cool water to your sinks on a warm summer day, condensation build on them just like on a cold coke bottle. Generally if you insulate the pipes it can reduce this from happening.
Another thing that is common to happen (and I apologize ahead of time because it is gross, but true) is mouse nests. Mice have been know to build nests in the ceilings and walls and to create moisture stains that look much like a leaking pipe. Check for any indications of mouse intrusion in your home before you pay a hefty plumber's bill
Another thing to look for is plants above. Sometimes as people water their plants, small amounts of moisture gets through and can accumulate into staining.
So in short, don't rush off to Google the closest plumber. Before you do that, check for those three items first.