Why are you ruling out AC condensation? If you ran the defroster, the
A/C compressor ran. And if the drain is plugged, the water may have
stayed in the evaporator case and only spilled out when you did some
particular manuver (a steep driveway, a particular curve at a
particular speed, etc).

I suggest you make sure the A/C drain is clear and monitor the
situation. Engine coolant leaks will not be palin old water, so you
should ease your mind in that regards. It is possible it is related to
a rainwater leak. When was the last time it rained in your area?

Ed