Yeah, I kinda realized that after I returned. I should have wiped it away before I left on the five hour drive, then checked it when I got back. I checked inside the valve cover as RRR suggested, and all was well in there. So I'm beginning to think that it's just condensed water that I need to drive out of the motor. BTW, this doesn't happen in the summer, leading me to believe that it isn't a problem with coolant leaking.