I can't help through personal experience but I have friends that had the same thing happen to them. They had more fish and the fish kept disappearing over a couple of weeks time frame. One day they were feeding the remaining fish and all of a sudden this 'thing' popped out of a hidden hole of a rock and grabbed the fish. It was gone and over in seconds! They didn't even know the 'thing' was in the tank, or that anything in the tank was large enough to eat their fish. They took the whole rock to the LFS
and the people there were thrilled to get this 'thing' as they'd never seen one in an aquarium before. It came in on my friend's live rock without their knowledge, of course.
I'm sorry I can't tell you what the 'thing' was because I don't remember what they called it. You may want to do a thorough check of your rock - every hidey hole. Could be you have a 'thing'.
I have put in a call to my friends and if they can remember what the name of the little beastie is I'll add that to this later.