[folivier]

I recently had the water pump changed since it started leaking. Since then I drove it to Cedar Key, then over to Silver Springs. Now I'm in Summerdale, AL about 900 miles of driving since getting the water pump changed.
A couple days ago in Silver Springs I noticed the coolant overflow tank was almost empty so I added 1/2 gallon coolant. When I pulled into Summerdale the coolant tank was almost empty again. I just looked all around the engine and hoses, in all of my basement compartments, and anywhere else I thought I'd maybe find a gallon of green coolant that leaked out. Nothing!
Engine oil looks good and hasn't been growing, tranny fluid and power steering fluid both look clean.
I was thinking maybe I have an air bubble that is just now working out of the cooling system but just want to make sure I'm not overlooking anything important.
I just added water to the overflow tank to see if it leaks out there while I'm sitting for a couple days.
Any ideas?