Went to top-off my windshield wiper fluid today and noticed that my coolant level has dipped below the min. mark. 2nd time, I've seen this. 1st time, they checked the system and topped it off telling me that there was not leak or other problem with the system. No charge, appreciated. This is the only vehicle I've had (OK unique in lots of ways) where I have been told that low coolant level necessitates a trip to the dealer. Wondering why and whether there is an alternative...like a source of the required coolant. Thoughts?