[Tommy]

I can only offer hope in the form of wild speculation. Different materials expand and contract at different rates with changing temps. I wonder if a gap between differing metals that is too tight to leak at "normal" temps opened up in extreme cold. . . If I couldn't find an obvious source of the leak, I'd top it off with coolant when ready to run, bring it up to operating temp and pressure, and see if any leaks appeared. If I saw none, I'd be happy.