Topping off the radiator does not necessarily mean it is full.
Leave the rad cap off and idle the engine.
Watch at the radiator cap and you will see air bubbles coming out of the system.
You may have a lot of air in there or very little but it could bring on your warning light if still low. Add your antifreeze mix slowly and when no more bubbles appear, put the rad cap back on and go for a drive for a while.
Fill the expansion tank too the full hot mark on it.
Your light might go off then or you might have a leak somewhere and loose the antifreeze.
Check it everyday to see how it goes till you see an improvement.