I believe the coolant loss is more due to evaporation than anything mechanical.
My BRZ does this as does my girlfriends 2016 Forester 2.5i. Her dad has a 2014 Forester 2.5i and it loses coolant at about the same rate.
All have a white residue on the overflow and all are filled with the blue coolant. I have a bottle of premixed factory coolant that I keep for them. I just keep an eye on the level and add as needed.
I have to catch times when her parents are gone to check theirs, they're bad not to check fluids regularly, believe the dealer has to do everything and must think anything you do will void your warranty. I put LED interior lights and weathertech mats in my girlfriends car and I heard about it!
