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.