As best I can recall, the amount of coolant that exists between the 'minimum' and 'maximum' fill levels is greater than what a can of soda would contain (even allowing for coolant expansion), which is probably why you have not seen a "Low Coolant" message. If the temperature was getting colder (going down below freezing), you might see the message as the coolant shrinks, but that is not the case at this time of year.
Perhaps ask your VW tech to have a close look at the cooling system next time you bring the car in for service. There are a few 'hidden' components - for example, the supplemental expansion tank that is directly aft of the main (spherical) expansion tank, in the cabin air intake plenum, and of course the auxiliary radiator.
When an oil change is done on a W12, it's generally good practice to remove the front right wheel and the front right wheel well liner. That is really the only way to get access to the oil filter without deforming all sorts of stuff. When that front right wheel well liner has been removed, it is then possible to carefully inspect the back side of the supplemental radiator for evidence of leaking. If there is any leak, there will be pink stains on the back side of the radiator - they are easy to spot, provided you have removed that wheel well liner. There are some good photos of all of the parts that I have referred to at this post: "Insufficient Coolant" Advisory Message.
Lastly - when the oil is changed on a W12 engine, it needs to be drained - it is not acceptable to suction the oil out, as is the practice with other engines. The Phaeton repair manual is quite explicit about this.