Coolant can leak from a number of places. Common leaks would be, Radiator leaks, Hose Leaks, Leaks in the engine-block core plugs, Internal leaks, and Leaky water pump.
Overheating the engine often results in a blown head gasket. The thermal stress causes the aluminum head to expand at a rate three times that of the cast iron block. This swelling occurs in the middle of the head, crushing the gasket, and allowing compression to leak between adjacent cylinders and coolant to flow into the cylinders.