Don't know for sure, but even unheated garages never get so terribly cold to make water freeze. For example, I buy bulk 500ml water bottles, and keep them in the garage, I'm yet to get a water from the garage (unheated) and find it frozen..
do you think theres 500ml of water in a big jug in your engine or is it in much thinner.smaller quantities? and surrounded by cold metal?