All of my Eastern garters also do that. In addition, I also find that they rarely ever defecate inside their hide boxes. Like all my corn snakes, my garters usually defecate in a corner of their enclosure near their water container, as far from their hide boxes as possible. Two smarter garters have figured out that if they defecate in their feeding containers after eating (I feed them in separate containers), they can REALLY leave their mess behind! ) These two regularly do this. My california kingsnake and my black rat snake also defecate consistently by their water container, away from their den.
My ball python however, seems to not care where she defecates. Maybe the ball python is unconcerned about cleanliness because like most birds, their feces drop away from them.
I suspect that other snake species who are ground dwellers are also clean and that tree dwelling snakes don't have this cleanliness instinct. Its a subject I've never seen mentioned in any scientific writing yet. Yet it's consistent-I've seen litters of baby garters be much cleaner about elimination habits than litters of puppies, yet we all know which species is considered more intelligent of the two!