I know this is a late response, but I'm hoping I can be of help.

The very first thing that comes to mind is Mantis Shrimp. If you have fish that are disappearing overnight, and there are no signs of any carcasses or body parts, you quite possibly have a Mantis Shrimp living in your rock. Another possibility, and one that is easy to check, is they might be in the overflow box...they can ultimately end up in the filter. My money would be on the Mantis Shrimp though.

No way your Horseshoe Crab or Chocolate Chip Stars are eating the fish. You do not mention having any kind of shrimp, i.e cleaner, coral banded, etc. The reason I mention this is because if you are having fish die, these shrimp could be eating the carcass but you quite possibly would see some remains.

Now back to Mantis Shrimp. These predators are common hitch hikers on live rock. They are a nuisance that can wreak havoc on a tank. I will not bore you with information on how to rid your tank of this creature as there is a plethora of information out there discussing Mantis Shrimp. Just do a google search. I will say this though...these creatures are highly intelligent, have great eye sight and will test your patience. If/when you attempt to capture it, do not sit anywhere hear your tank. He will actually see you before you see him and probably will not come out to get trapped. And if your first attempt is a failure, future attempts will increase tenfold.

Good luck.