Each year when the temperature drops below 10 degrees F, I notice that
the level of antifreeze drops more than usual.   This has occured on the
last 3 cars I have owned.

I drove the car on a 7 hour trip and the antifreeze level stayed the
same.   Within a week the temp dropped to 6 degrees and the antifreeze
level dropped a couple of inches in the over flow.  There is no
antifreeze under the car and the heater core was just replaced and it
doesn't smell like antifreeze or steam.

So, why does the antifreeze level drop significantly on the coldest day
of the year?