dex-cool coolant leak

I have a 2002 Chevy Venture minivan with 52,000 miles. Recently it started to leak dex-cool coolant at night when the temperature drops down to around freezing. In the mornings, there is a small puddle of coolant on the garage floor near the center of the engine (slightly to the passenger side) of where the oil pan drain plug is located. To my knowledge the leak does not occur in the daytime or while the engine is running. It is hard to see where the leak is coming from but it appears to have dripped down the engine block from somewhere. It is not a the radiator hose. Please help!