"low Coolant" Message On Cluster

[Old Dave]

We bought a 2001 Yukon XL (98K 5.3L) a couple months back and periodically we get a "Low Coolant" message displayed on the message center on the left side of the instrument cluster. The level is fine in the tank, truck is running at the correct operating temp. and it runs fine. This doesn't happen all the time, just occasionally and seems to be only in town. It came on when we coming home Mon. night when I was sitting at a light. It stayed on all the way home (25 miles on the highway). When the light comes on and you shut it off for a minute and restart it the light is off. Sounds like some sensor, maybe in the overflow tank (radiator) that is not working correctly but I haven't investigated that yet. I called two auto parts and they don't show anything. The dealer said there is a sensor that is part of the coolant surge tank (overflow bottle) but it's about $60. Anyone been down this road before?