I've had fish disappear in the past if it didn't jump out or get sucked up into he sump , it could have went behind the rocks and died , I've seen many people tear apart there tanks to find the body with no luck, 90% of the time if a fish dies behind a rock the clean up crew crabs snails worms ect will take care of it almost instantly ,
if I can't find a fish I assume 1 of 3 things , I don't stress over it any more just let nature take it's course ,
I refuse to rip apart my tank to find a stupid fish that most likely died and was eaten ,
because it's not worth the head ach ,
I have bought fish online to have one or 2 die off once in a while , the vendors want a pict of dead fish , I explain I'm not ripping my reef apart for a fish and take a picture of the tank showing why , if you deal with a good vendor they will understand , most of the vendors know me by name as I deal with them a lot over the years , I spend way too much money for them to deny me a credit for a dead fish , and a fish isn't worth loosing a good customer over , as a few vendors found out .
sorry I went off track a little