I've been running my truck for about 13k miles without any issues and all of the sudden my low coolant indicator comes on one morning. I changed my oil today and it looked good, can't see any wet spots anywhere. I have a merchant alum coolant tank and the coolant was barely visible in the bottom of the tank. I added a gal of 50/50 I had laying around. Would anyone have a guess to why or where I'd be losing coolant?