Any chance there wasn't enough antifreeze in the coolant mix so that a small amount in gasket pocket could have froze/expanded gasket enough to cause leak?
I'd look for an odd colored plug/plugs and perform a leak-down test or just hook up some compressed air(120psi) into spark plug hole(piston at TDC on firing stroke so that valves are closed) and listen for noise in intake/crankcase or bubbles/pressure in radiator.
Hope its not, but you should check if you can't find a vacuum leak on intake somewhere IMO.
I mix my coolant to water 60/40 so there shouldn't have been to much water, although for about three weeks we were sitting at -10 in the mornings and 15 all day, so it was pretty cold. I'll be checking the plugs in a few minutes hopefully. I can't do leak down test right now, but I'll try to do a compression test soon. And I originally checked everywhere for a vacuum leak with carb cleaner, but found nothing.