Where could a leak be coming from the engine coolant?

If you can see no external leak, then suspect an internal leak as in a Blown Head Gasket. This is very serious and severe engine damage will occur if you continue to drive the car. Get this repaired ASAP, and stop driving the car.

You do not let us know if you see coolant on the ground or on the engine. It's hard to diagnose with out a little more info.