it is possible that if one of your clowns died , the bacterial and invert inhabitants would quickly make claim and have a feast, thus leaving nothing to be found. check behind rocks for a body.
the same happened to me, but I found him dead between the rocks ,I think the culprit was my three stripped damsel, he has been banished to the sump since then....... If anyone wants him its yours free, I will post him later in the freebie thread.