I got myself a small/medium tub freedom breeder rack about a year ago and shortly after moving the babies and juvies in there I noticed a couple of them had the same symmetrical spots. I would see them all upside down rubbing their chins, so that seemed like the cause. As they grew and shed and some moved up to larger tubs, the "holes" seem to go away. They have not seemed to affect any of my snakes negatively, I would say about 7 out of 25 snakes in that rack have them.