I finally got around to hacking back the grass behind the house and found a hole in the ground by the wall. It is a large hole, about the size of a rabbit hole and I have noticed the complete absence of rabbits around here.
The gf said 'snake'. I didn't know they can excavate such a big hole. Anway, this morning I looked out of the kitchen window and saw the snake, which disappeared into its hole after I alarmed it.
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Probably the same Keeled rat snake I saw a while back, although it looks longer. A good 2m long and it moved quickly once it decided to take off.
If it is a rat snake, what are my options? My main concern is the gf's small dogs, which have no track record with big snakes.