Low Coolant Level

Every time I start the car it gives me a message in the info center that the coolant level is low. It has been going on for 6 months and coolant level is not changing at all. I remember when I had 97 Buick Park Avenue and the same problem was caused my faulty coolant sensor in the radiator. I looked if there is a sensor in the radiator but I really don't see one.

Where are coolant sensors on 2005 CTS with 3.6L engine or is there anything else going on?

I searched this thread for "coolant" but not much here, I can't believe that I'm the only one with this problem.

Any help would be greatly appreciated.