2003 7.3 - Mysterious Disappearing Coolant?!

[AZFords]

Great forums here, I've been a lurker for years but this issue has me completely stumped (and my mechanic as well).

I have a 2000 7.3 in an Econoline that is mysteriously loosing coolant. A lot of it, around 1 - 1.5 gallons every 180mi trip.

A little background: it has 230k miles on it, runs like a champ, and had it's front cover gasket replaced less than 1k miles ago to fix coolant leaking into the oil pan and was then not driven for 9 months. During that fix they checked and tested the oil cooler and pump as well.

There are NO visible leaks under the vehicle (no spots, no puddles), no white smoke, no coolant in the oil, no oil leaks, no antifreeze smell in the vehicle, and it does not over heat. The same shop that replaced the front cover (excellent staff and very knowledgeable) has pressure tested the system multiple times and found only one thing wrong - the thermostat was loose and needed to be replaced, which they did. Every time it goes out it consistently uses 1 - 1.5 gal of antifreeze, but runs fabulously.

I've let it sit and idle to heat up and the level in the reservoir never changes, no leaking, no seeping around the water pump that I could detect.

Anyone have any thoughts about this? If more info is needed I will be happy to supply it.

Thanks in advance for your time!

Jason