I have had Rics go missing before in my tank. In my case they were not happy with where I placed them (to much flow). They let go of the rock that they were on and ended up in the rockwork at the back of my tank. Not sure if this is what is happening with you but it is a posibilty. It could also be a crab and an enuice worm could be the culprit that is seen in this thread.
http://www.canreef.com/vbulletin/showthread.php?t=46894
Chris