So, I was adding some wiper fluid Sunday when I noticed that my coolant reservoir was low (ie- empty) in the holding tank. I had some pre-mixed coolant on hand so I dumped what I had in there which got it back up to where I could see it in the reservoir but not all the way back up to the cold fill line. This morning (Tuesday, after driving the truck to work, lunch and home yesterday) I looked and the reservoir was empty again. I mixed some coolant & water 50/50 and filled up to the fill line. Planning to check again tonight and tomorrow to see if it keeps disappearing. I've seen no puddles in my driveway or where I park at work, so I'm thinking maybe it only leaks under pressure? Any thoughts on what might be up?