Thanks, Vette. Keep us posted when you know more.

It has been a long time since my last update here. The reason... The problem went away. I have driven about 2500 miles since the last post and no more coolant has disappeared! I have not been able to detect a difference in the level in the expansion tank and the radiator has remained full. Now I am scared to take the car out of state. I have made relatively long trips, ~120 miles and many short trips of 20 miles without the problem returning. If anything changes, I will post it. Thanks to all who replied.