Antifreeze Disappearing

I have read most of the former posts on this subject and don't see an answer that fits my problem.

I have a 1994 E320 wagon wiht 129,000 miles on it. I recently had a major service (timing chain, head gasket, radiator flush, etc., etc.) Shortle after that service, I noticed that I needed to add about 16 ounces of antifreeze to top-off the coolant tank. I had never needed to do that before. This was about 1,000 miles after the service. On a recent trip of about 250 miles, I noticed the coolant-level-lowwarning light come on and the temperature start to rise. Fortunately I was near home whan that happened. Upon cheching the coolant level, therer was none in the tank. I needed to add almost a gallon to bring it back to normal.

There is no smell of antifreeze anywhere, no aparent leaks or puddles, no steam from my exhaust - it's just disappearing. I've been watching the level daily and it's still SLOWLY going down again.

What could this be (I suspected a possible new head gasket problem) and what could make so much of it go-away so quickly and then almost stop going-away?

I really appreciate the expert opinions and help from the pros on this forum.

Thanks again in advance,

Jack