There is only two places coolant can leak. External or Internal.
If there are no external leaks then it has to be internal.
The best way to check for an internal leak is to complete the following procedure.
1. When the engine is cold, pressurize the degas bottle with an appropriate tool to normal coolant pressures. Typically 20-21 PSI.
2. Get the engine to operating temperature with the tool still installed on the degas bottle.
3. Once at operating temperature, shut the engine off and allow it to cold soak over night.
4. The next day, remove the spark plugs and use a bore scope to inspect the combustion chamber.
If there is coolant pooling in the cylinder then there is an issue with the head, head gasket or the short block. The head will have to be removed for further inspection for cracks, warpage, etc. There is a procedure in the WSM (workshop manual) to check for head and short block flatness. The head can also be pressure tested at a machine shop but in my opinion, that is a waste of time. If there are absolutely no faults with the head, gasket or block then I would replaced the head and the head gasket as they are the most likely.
If there is no coolant in the combustion chamber then remove the down pipe from the turbocharger and check for signs of coolant which will be a white residue
If coolant is found then either is an internal leak within the turbo charger or there is a porosity fault within the head. If the turbocharger is at fault, there may be signs of coolant in the CAC (charge air cooler) tubes in which case the turbo charger should be replaced. If there is no coolant in the CAC tubes then the head would be the most likely as it is a sign of a porosity concern.
This is how your dealer should be checking for coolant loss, if they are not then they are most likely wrong.