Mystery leak

Hello, just joined the forum. Have a 2006 2500 diesel automatic trans. All fluids are full and can see no signs of a leak but when I park the truck for a day I find a 8 to 10 inch wet spot under the engine kinda in the center. The spot will dry up and leave no stain. Took it to a few shops and coundn't find anything. I think it is coming from the fuel filter water drain switch but can't see it leaking. Anybody else have this happen to them. Thanks