Are their shells laying around? Or are the shells missing too. If they are there, are they in one piece or broken up. If they are intact it could just be the normal struggle that hermits go through to get bigger shells when they grow. If they are gone or broken they you might have a predator. I've never had to deal with a mantis shrimp but there are lots of plans for mantis traps on-line.
Welcome to
Talking Reef! I hope this helps.
Jimm
