Coolant leak 1992 toyota pickup

I have a 1992 toyota dually pickup with a 3.0 v6 engine , i just recently had the head gaskets replaced at a toyota dealership,after driving 400 miles a coolant leak started at the rear of the engine. i took the truck back to the dealership and they replaced the headgasket and bolts on the passenger side.i drove the truck home and after about 36 hours another leak appeared at the rear of the engine ,the leak only appears when the truck has been parked for 24 hours or more.i also noticed that it happens when the temperture gets colder. they have checked all the hoses and freeze plugs and the pipe that goes under the intake. Mark