About two weeks ago I noticed that my coolant reservoir was almost empty. This was the first time I noticed any coolant loss. Of course the first things that popped into my mind was head gasket. But there is no sign of that, no white smoke, no coolant in oil. I filled the reservoir up and watched it over the last two weeks. It went down about 1/2 inch, with about 200 miles driven. Next thing I checked was for signs of coolant around the radiator, hoses, and water pump, but again, nothing. So I am a bit at a loss. It's loosing too much coolant to just top it off regularly, but I have no idea where I am loosing it. Any suggestions? This is a V6 with almost 190,000 miles.