I think my purple firefish may have died in my tank. Over a week ago, I saw an isopod attached to him. Each day after that, I noticed what looked like scars on the fish and the fins looked ratty and worse each day. I don't have a QT tank so i had no where to put him. Last I saw him was Saturday morning and he ate. Is it possible he died under the liverock? i don't want to tear down the tank to find out. I have not seen any amonia spike. the only other fish, a clown, seems fine. Should I be real concerned, or did the CUC take care of the body? This is a 24g Nano Cube

If the bodies are accessible I usually remove them. Otherwise the hermits will take care of it, everything actually will.

It's okay if you can't find it. However, you should be concerned for the clown. Look here for more info:
http://reefkeeping.com/issues/2006-02/bp/index.php

Firefish can hide for days at a time. I've had mine MIA for a week and then he'll just come back out like nothing ever happened. But they are also known to jump when in distress. Have you checked the carpet behind the tank?