I'm not excluding the possibility that your HG has an internal leak, but the design of our gaskets makes that seem unlikely.
The portion of the gasket that surrounds the cylinders is lined with metal which should keep them from failing internally. The biggest weak point of the design are the extreme ends of the HGs (where the red sealant is in the photo) - there's literally 1/8 of an inch of gasket between the water jacket and the atmosphere. This is why most of our leaks manifest themselves as slow drips out the back of the motor.
There's also the possibility that you have a cracked block that's leaking past the liners, but that's usually not the case unless there's been a serious overheat at some point in the past.
My money is still on some pinhole leak somewhere that stays hidden until the system is under pressure. Double check the area on the very top of the radiator end caps - you can only see it after you remove the shroud. My driver's side cap had a tiny pinhole leak that would only be visible after it was up to temp and even then it would evaporate before it hit the ground.