Heather,
This looks very similar to something I have seen alot in my hatchlings up to sub adults. My first question to you would be what type of rack are they in? I ask, because almost every hatchling that I have in my ARS racks have this (yet the ones in my homemade melamine racks do not). My theory on it is that the snakes rub on the holes on the metal that forms the top of the tub in a pre-fabbed metal type rack. I have often opened tubs, and found the snakes with their heads up to the top of the tub as if testing the air coming through the holes. I have found that once I move an animal out of those type racks, and put them in CB-70's (homemade racks again that do not have metal tops), that they marks go away.

Let me know if you need pics, and I can take some pics of that occuranc on some of my animals.

Hope that helps,