2000 Buick Century Coolant level

I recently took my car in for an oil change and the mechanic filled my coolant reservoir tank with coolant. Since he did that, my coolant light comes on for about 1-2 minutes or so every time I start my car. After a minute or two it goes out. I added a little water just in case (about a quart or so) but it still comes on. My temp guage is fine, the car is not overheating in any way. Can you tell me if perhaps it is the coolant level sensor? If so where is it located and how can I check?
Thank you in advance,
Helen