I've been losing coolant for as long as I've been checking my coolant (since last summer). I want to fix the problem. About two weeks ago, there was nothing I could see in the radiator cap and nothing in the overfill bottle. I've never noticed anything weird about the water temperature, and the heater stays comfortably warm when the temperature is 20 degrees Fahrenheit outside (once warmed up).

Today I filled the radiator cap to the top, and the overfill bottle to the max line and let the car heat up. I'm not leaking coolant that I know of, I've never seen any on the ground.

Where is all this coolant going?