In answer to your question,....it could be that he's hiding, or fell victim to a predetor. I bought two pepermint shrimp, dumped them in my tank, and now I have just one. Don't know what happened to #2. But the one that I do have likes to hide behind rock. They are nocternal, so check your tank out with a flashlight, after the lights have been out for a while. There's a good chance he's still around!