Welcome to the wonderful world of the waterboxer.  On each of my three
waterboxers, extreme cold showed latent defects in the head gaskets.  What
happens is that the cold rubber shrinks, and any minor leak shows itself (in
one case I had running coolant when starting on cold days, but the leak
would stop in warm weather.  It was another 2 years before it got bad enough
to have the head gaskets replaced.

Generally in warm weather, there is a leak,but it is so slow the coolant
evaporates before you see it from the bottom of the engine.

Good Luck

Scott Grillo
'91 Carat