Go

[hummer]

I've had fish hide inside holes in rocks before but if you've checked them all then there's a chance that it died and your clean up crew ate him. One of my small clown fish went missing last week as well. I've looked all over the place and couldn't find him. Haven't seen him since.
If you have a sump and there's no mesh protecting fish from jumping into the overflow then you could check inside your sump. If you have filter socks you might want to check in there too. <-- Opps you said you checked this already.