I agree with Mike J, I would stalk the tank at night with a red light, maybe even put a chunk of shrimp on the sand to entice whatever may be hiding in your rocks. I had a tomato clown, coral beauty, and a sunrise dottyback disappear in my tank over the course of a few weeks and figured they just died and my CUC ate them, those were the only fish i lost. After my tank broke a few months later I finally found the culprit, I had a small mantis shrimp about 3 inches long. I also had a pistol shrimp so when I heard clicking in my tank I assumed it was the pistol. Just sharing my experience, hopefully you find out what the cause is before losing more fish.