I've only ever found ONE cadaver of a fish that died in my tank and I've had many disappear forever. (Well, not many but quite a few.) All else have just vanished into thin air. I've theorized what everyone else is saying--that the clean-up crew devours the fish as soon as it dies and you just never see the bones.
Like jadams said, clowns are very territorial. Heinously so. I had a pair of clarks and the male died, so I put in another smaller one (a smaller one in a group at the marine shop), and she chased it non-stop until it 'disappeared' forever. A week or so later, I added another smaller one. She dive-bombed him, too, for about 2wks, but he managed to survive. Eventually she accepted him, but it was a longggggg ugly process.
I wouldn't give up entirely on acclimating another fish to the one you have, but there might be some collateral damage before you land the right fish to withstand the pressure long enough to be accepted by the resident one...
Good luck!
I'm with the rest of the folks. The smaller ones were prob chased to death and eaten by the cleaners.