A weather bulletin prompted me to go outside to the enclosure and make sure the turtles had cover. None were exposed, but I found one in the little pond, completely submerged in 49 degree water (air temperatures are higher than that, for the time being).
I really don't know how long he's been in there, either, but its interesting to see where they seek shelter from extremes in temperature, especially at this time of year.
Stephanie