I have a 2017 Subaru Forester that has low coolant in the reserve. It fluctuates high to low and I and service dealer have added coolant/water accordingly. Some passengers have smelled coolant. Dealer has twice inspected and used some type of pressure and found no leaks. This has been a problem since I bought it new. Any ideas as to what is wrong.
Try a hot pressure test. you may have a gead gasket that only leaks at operating temp. Also, you might want to try a chemical test to look for exhaust gas in the coolant. Keep all your records on this.
Be sure not to do anything to void warranty, With dye in coolant, You can take some solder wire twistie tie material or any thing thin and flexible, attach a small absorbent material on the end, stick it up the HVac drain. If you can find moisture on the end check for coolant with black light indicating leak in heater core.