Just because you "don't see a leak" do not think that you don't have a leak. Unless you crawled around on top of the engine and looked deep in the valley, looked up the back of the engine on both sides and inspected BOTH of the radiators and peeked at your HVAC box drain you have not looked. Coolant can be deceptive and elusive so it is helpful to know that the Motorcraft Orange coolant will leave a dry white trail. Head gasket failures generally do not occur on the 6.7L PSD. If you don't find a leak externally I suggest that your EGR cooler would be the next likely place to look. While the coolers on these engines are commonly clogged with carbon, they also generally do not leak but they have been known to! Even if your EGR cooler core has been replaced, perhaps for an emissions concern, do not assume that "new" means "you're good!" An internal leak could be the result of the rebuild process and assembly when seals and gaskets can be damaged or fasteners improperly torqued... I am just rambling on here. I'll shut up now.