Last night when running the heater it suddenly blew only cool air - I drove about 2 miles after that. (I thought: coolant?). Today I added about 3/8 gallon of coolant (50/50) and now the heat is back. However (because it has been a snowy and wet day I can't locate any leaks or even the odor of anti-freeze). The question I have is: do I have a really slow leak or is it likely that I have a fast leak as evidenced by my sudden condition of losing my heating? It is night time now and 23 degrees F. so I can't really check the ground much for a slow leak.

One other clue. About 2 or 3 weeks ago I noticed that the radiator fan came on when I shut down the car and the weather was pretty cold. Maybe 40 degrees. That seemed odd since this normally does not happen. I wondered about some maybe faulty sensor telling the fan to come on. The temperature gauge has alway showed normal (straight up).

Today I looked for wetness along the hoses I could easily see and also around the expansion tank. I don't see any obvious suspects. I do not smell antifreeze in the cabin.

Thanks.
Any other ideas?