I guess it depends how much antifreeze is in the car, how cold it gets, how long the car is left without running. I have a bucket of water in the garden that had a nice layer of ice on the top a couple of days ago when we got -2. I suppose you would be unlucky for it to freeze enough to do any damage but I'm not an expert, the only car I've seen pop a core plug was my dopey brothers many years ago.