Silverado coolant leak
 

I have a 2008 2500 Silverado 4WD with a gasoline engine. I have an unexplained intermittent coolant leak when the vehicle is parked. Occasionally, when the truck is parked and the engine is cool, often not driven for a day or two, I will discover a puddle of coolant on my driveway. A 24 hour pressure test of the system found no problem. It is leaking from the driver's side and drips off the frame. Anyone else ever see this and were you able to find the problem?