Loss of coolant

[North Bend Stroker]

My 05 6.0 seems to be loosing coolant. The truck steams out the exhaust in the morning & evening for about a mile or so ( bluish-white steam) that evaps quickly, then the steam stops all together. The degas bottle will loose about a 1/2 gallon or so in 3 weeks , can't find any external leaks or spots on the ground. I have a scangauge II on board and have no fixed or pending codes. The truck seems to be running fine, no power loss or idle problems.

The dealer had me slightly overfill the bottle & mark where I filled it so a time & loss measurement could be taken. Not sure why?

EGR cooler? EGR valve? Any help would be appreciated very much!

PS
I have just over 78,350 on the truck.

Thanks,
N B Stroker