I know this isn't classy but - smell it. If it's coolant it will smell sweet. If it smells like detergent it might be the washer bottle (don't use detergent additives in your washer bottle people, it will eat the rubber seal...) Chances are you did spill a little coolant and the run off finally trickled out of the fins and off the bottom tank. Roll the truck back and see if it creates a new pool on the driveway overnight/during the day.