When my adults pull their earthworms out of the container and into the substrate, I've noticed they back up as they work on swallowing the worm, and as they back up, that also helps to wipe the substrate off the worm. I've noticed the babies back up while swallowing too, even though there is no substrate in their feeding containers. I wonder if the backing up behavior is a garter's natural way of trying to clean debris off of a meal as the snake swallows it.
As for the escaped flame, it may be hiding right under your nose. As a veteran of hunting cats hiding in the house (and cats can disappear in an empty room when you're looking for them), the one thing I learned that has helped me best, is to think like a cat, to look in the kinds of places cats like to hide, etc. Cats can think of some places people don't think to look in, such as inside the upholstery on the backs of chairs and sofas, or inside the thin fabric that lines the bottom of box springs (and always sooner or later tears away from the frame, giving cats an ideal hiding place). Think of where you would go if you were a scared garter snake, and look in such places. There are some good articles on finding escaped snakes on the web. Luckily, the only escape I've had in recent times, was when the smaller adult garter managed to push up the lid on a 10 gallon tank whose locking lid's locks didn't quite do the job. I panicked and was reading articles on the web for ideas, then I walked back over to the tank to make sure the bigger snake was still in there. Beside the top of the tank is a shelf with a bowl of crystal rocks in it. As I reached for the tank lid, a little head stuck itself out of the bowl of rocks as if to say "look what I did!" I picked up the triumphant little escapee and returned him to the big tank which has much better locks on its lid. If he hadn't reached out though, I may never have noticed him hiding behind the rock bowl on the shelf. So don't give up on your flame yet (and it would make me absolutely sick to have something as beautiful as a flame get loose and disappear!) I hope you find your flame escape artist.