My coolant light would come on randomly, even though everything seemed fine. After a couple of weeks, I realized I was VERY SLOWLY loosing coolant, but couldn't see it leaking from anywhere. My alternator bracket/waterpump bolts had broken, and coolant would leak from the waterpump, but only when I was a speed, and the wind would blow away any of the drips. Took me a couple of months to figure that one out.
