I had this issue when it first started to get cold and my level was near the bottom. Adding some distilled water was adequate and I haven't had that problem since then. That being said, the cooling system is designed be completely contained and theoretically not lose any fluid level unless there is a leak. If this is consistent, even later on, check for leaks. You will not notice a puddle of coolant under the car unless your belly pan is removed or you are gushing coolant from somewhere.