Sounds like a common issue. 
The o-ring on the drain valve goes bad with age. 
It only leaks when cold or very cold. 
The good news is it is a very easy fix.   
one screw, pullout the valve, keep an eye open for the spring and ball. 
Clean out the bore, install a new o-ring and reassemble. 
Skydoc_17 has a kit with the o-ring and a new stainless socket head screw.