I have to say, I side with the person who says it may be a scrape from escaping his enclosure--a loosely latched screen top, for example. Notice how the scarring is over the widest part of the snake's body, and runs the length of the top of the spine. You can also see how it fades rather than stops both at the beginning and the end--lighter scars being seen at slightly smaller diameters of the snake's body. While rodents will bite and chew at the same area, the fading out on either end of the scar is less suggestive of that--at least in my opinion.

If the snake were most of the way out when the damage began, he could pull back, but it would probably be unsuccessful--or he could instead choose to continue forward, since he's almost out anyhow. That might be how he wound up in a back yard in the first place.
It looks like it was a pretty nasty wound, either way, but it looks like it's fully healed now.