If you have been losing coolant slowly but steadily and the oil on the bottom side of the oil filler cap is a creamy color, there is a good chance the intake manifold gasket is leaking.
Does the coolant slowly disappear from the pressurized reservoir? Any over-heating?
If this is the case and you do not see any exterior leak, the coolant may be leaking to the inside and that can become VERY costly. If you suspect this have a coolant pressure test done to verify.
These engines haven’t had this problem anything like the V6s.
Because you said it runs fine after it warms up leads me to think you do not have an IMG leak.