I've found both Nerodia (water snakes) and Crotalus (rattle snakes) out in the high 30s to mid 40s. It's not common but it does happen. I know other people have found panthertopis (rat snakes) and thamnophis (garters) out in equally cold weather. Again, not common but it does happen. You haven't lived till you step nearly on a rattlesnake in a cold day in november only to have it crawl over your foot to get away while you're thinking "OHCRAPOHCRAP". FWIW, I was out helping my wife collect plants for a plant classification course...hadn't expected to see anything herp related...
The other chance, is if you had a few warmer days (not very warm--just up to the low 50s will do it) followed by a quick cold snap, it might have gone out to bask and warm up, and gotten caught out when it turned cold too quickly. Leaves them easy picking for predators like 'coons and hawks and dogs and skunks.
The Third option; if the snake had a preexisting injury or illness, they can become very thermophilic and go to great lengths to get *any* additional warmth; if air temp was 40, and there was a sunny patch that was 45 or 50, it wouldn't be too abnormal for a sick or hurt snake to try to utilize that for warmth, even if air temps are lower than they'd usually go out in.
-----
We wouldn't have 6 and a half billion people if you had to be beautiful to get laid.
6.6 African House snakes
3.2 reticulated pythons
.1 corn snake
4.2 Florida Kings
1.2 speckled kings
1.2 ball pythons
0.0.1 Argentine boa