Thanks for the reply Amphi... I hadn't had a chance to take any pics, and when I tried to, the clown was gone. I'm pretty sure it's dead, as it's always easily visible. I have lots of rockwork in there and by now it's probably been eaten by the many critters I got in there.
I have no idea how this could have happened, as I haven't introduced anything in the tank in months.
In any case, it's a sad loss, was a very nice black ocellaris... Hopefully it isn't anything that will have an effect on the other fish in the tank