C230 Coolant Level Mystery

[BuckNecked]

1999 C230K (202.024) Transparent cooler reservoir with black top. I keep my coolant about 1/4" below black top so I can monitor level. Drove 400 mile trip (used heater for about 5 minutes) and level was 1/4" low when cooled off. Stayed that level for a week. Found no leaks. Drove 10 miles today and coolant was back up to the original top mark. Loosened reservoir cap and coolant came up even more (lots of air escaped). Where did this excess coolant come from or where has it been? I changed coolant 6 months ago. Could air have been trapped in system somewhere? Thermostat problems? How do you diagnose thermostats? I certainly don't want to "cook" this engine. Your input is greatly appreciated.