I'm still convinced that it's an Anghelism for some sort of refrigerator or cold-storage unit, like the kind Ryouta and Hiyoko hibernate in at the end of BBL while waiting for medical technology to improve. (Of course, since said unit is inside the abandoned medical facility, we could very easily both be right.) It has to be more than just a traumatic flashback to a time that officially ended with Nageki's suicide/burning down the entire facility five years ago, since Anghel is still a missing student during every otome route except his, possibly the dismembered student at the end of Shuu's route, and only makes any sort of appearance if you wish for the mad love of a fallen angel and/or make it to about 4th class of BBL. He's still being kept in the Cage of Ice right up until the present day, obviously long after the medical facility shut down.