It's amazing how small fish can hide. I bought a couple of very small clown Loach back in 2006 and we couldn't find them anywhere one day, I took everything out of the tank and they still weren't there. I actually had a fake piece of root in the tank, made of some kind of plastic stuff. Anyway, there was a tiny hole in this fake root and the Loach were managing to squeeze through so they were inside it. So when I actually took everything out of the tank, they were still inside this root

it's amazing they didn't die.
So my advice to anyone who has got small fish that have gone missing, look in places that don't seem obvious, the chances are, they've squeezed in somewhere to hide.