Don't just look at the bottle and see green and think it is full. The plastic overflow bottles will stain over time and if you don't physically look, it may look full when it is empty.
Or you could just have the typical dead sensor inside the tank. If you normally keep your eye on your fluid levels you can just unplug that sensor and the light will go off.
I doubt the sensor is bad. If the light is on, than your coolant level is probably low.
When I put the aftermarket coolant expansion tank on the cobra, there wasn't a place to put the sensor, so I just left it disconnected. The light remains off when the sensor is disconnected.
thanks to all who replied ,, I just did everything mentioned from pouring a lil more anti freeze in, clean the sensor and the three prongs,.. no more light... easy fix... thanks again everyone