Exhaust systems have small drain holes in them so that the water does not accumulate and rust things out. Usually there is one at either the front or back of every muffler. Especially during humid or cold weather you may be inclined to see more moisture.
Ford forgot to include drain holes on 1991 Lincoln Town Cars and they had to replace a lot of mufflers under warranty on those cars. The blown out mufflers made those Lincolns sound like hot rods.
