It is a possibility that you have a air pocket in your coolant, it is more prevalent in the cold, and only shows up while you are driving. It does not take much for the reservoir to give you the low coolant message.
To take care of it, if it is what the problem is. Pop the cap off of the crossover tube just in front of the alternator. Start the car let it idle until the thermastat opens and cycles the coolant. If the reservoir dips then you have an air pocket, just simply add some coolant in the cap on the crossover tube and you should be golden.
Hope that helps. :Beer