Their is a simple hydrocarbon test that any shop can do that will show a coolant leak from the combustion chamber. Its cheap and it works every time. The only thing is getting it up to operating temperature with the fill cap off. The way it works is their is a test tube that comes in a kit along with a blue chemical, you put the chemical in the tube and it then goes over the fill where it filters the fumes coming out, if the chemical turns light green or yellow their are hydrocarbons in the cooling system where their should be none. I have used this hundreds of times with great sucess. The only thing is on small leaks it takes some time, some heat, and some cylinder pressure (ie; revving the engine) to get the leak to show. The only reasion a dealership would not be able to find a leak, would be not doing the test, not taking enough time doing the test, or not wanting to find the problem... Here is a vidio of the test I found on utube, as you can see its not rocket science. How to test head gasket failure using an antifree…: How to test head gasket failure using an antifreeze HC Tester - YouTube.
Mark
Mark