2001 Ford Focus leaking coolant

The last two nights it went below freezing (in the 20's) my car had a large puddle of coolant running out from under it. The next three nights were much warmer (lows in the 40's), there were no leaks. I started the car, after bringing the fluid level back to normal, and let in run for an our. I could find no leaks. I drove the car for about 5 miles and still could not find any leaks. Any ideas? Thanks John