How low was it on coolant? Was this the first time you checked it or was it low in relation to the last time you checked?
Coolant tank failure would leak into the trunk. If it leaked out of the overflow (located above passenger rear wheel), then there was most likely an air bubble in the coolant system from you topping it off. This happened to me and you just need to refill the car with coolant, it's 50/50 coolant/distilled water. You can take it to your dealer like I did when this happened, but I paid $700 to be told it was an air bubble.
