cap or tank
Sorry I forgot to mention to check the cap.
This type of system is new to me...a former tech. I looked at my car and noticed that there is no coolant overflow tank as previous cars of all makes have had in recent years.
I also noticed that now, when car is cold, I can squeeze the upper radiator hose and hear air going in and out of the radiator cap. That means the system is open to atmospheric pressure when coolant is at outside air temperature.
A failing cap or plastic tank which does not properly seal (threads) the cap could be an issue as others here have suggested. Old coolant should make no difference in whether a system leaks or not.
Ford suggests replacing coolant every five years. Time for me to do it also. I need to do mine but don't trust the dealer to get all the old coolant out of the system, so will do it myself, flushing with water completely (while thermostat is out of the car) and draining before replacing with new coolant.
Others in this forum, in the past, have recommended which coolant to use. I cannot recall what they recommended. You'll notice in pics of new Mustangs, that Ford now uses a Red coolant instead of the Yellow used by the Factory in our cars.
I do not know the difference. Maybe Prestone Green is o.k. also.