Since the winter of 2009 – 2010 has been very cold it is no surprise no activity was observed. The terrarium was covered with snow for many days and the pond was almost constantly frozen from end of December onwards.
All one can do is hope the snakes have been able to crawl away deep enough in the hibernaculum (or underneath the water in the pond) to be below freezing levels.
I have not attempted to check this, but since it is probably the coldest winter since I built the terrarium it will be a good test.
Both my hibernaculum and the pond are not heated and about 80 cm deep.
In previous years there have been occasional sightings of male Thamnophis sirtalis on a mild January days.