Darkfrog,
Sorry to hear about your losses. I had a small hitchhiker crab come into my tank. He must have been in the tank for close to a year before I knew he was there. One day I saw him and tried (half-heartedly) to get him out (he was so small and cute looking), I couldn't get him out of the barnacle that he was in and really didn't want to kill him, so I put the barnacle with crab back in th tank (big mistake). After a few months of watching him grow, I was becoming more concerned but still did not remove him.
One morning, my 6-line never showed up for breakfast. After a day or two of searching, I finally found what I think was his tail. The only thing that could have gotten him was that crab (only about the size of a quarter). I then started looking for him in earnest. Eventually, I found him under an old dead bleached out clam shell that I keep in the tank. I pulled the clam shell out of the tank looking for him and almost put it back in the tank because I couldn't find him. Fortunately, I did see him mixed in with some stuck-on sand on the bottom of the shell. I did remove him, then.
Point is, they can be very hard to see AND remove, even if you have the right rock.
regards, fsa