I live in between Springfield and St Louis. I guess that technically makes it more southern than central Illinois.
I saw it again today. It seems to have moved into my yellowfoot's pen. I found it coiled up next to my male yellowfoot. At least I think it was the same snake. It was the same size and same coloring. It will be interesting to see if it stays. I have lived here for 6 years and this is the first year I have seen any snakes.