I found a juvenile black rat snake in my foyer a few years back. Since I have garters about the same size I did think that one of mine got out. It wasn't until I had it in-hand that I realized it wasn't mine.
Hindsight being 20-20 I should have kept it and chucked the garters out the door (they are native) and kept the black rat, it was a very chill critter.