How is your coolant level now?
There is a sensor in the expansion tank. The coolant level needs to drop below it in order for the light to turn on. If your coolant level is good, then you need to check the sensor. This is very rare. You probably are loosing coolant. Loosing small amount of coolant is usually a sign of a bad water pump. This will only get worse. Check the wheep hole for signs of coolant or crust.