I have seen personally an emerald crab eat a cleaner shrimp. Now I am not sure because a person's tank that I know, he prolly wasn't feeding the crab alot. but I have seen the crab grab the shrimp, shred it and eat it and all that was left was the crabs head. just my .02
That could be the culprit or like stated by others it is hiding during the molting process.