My pond is up on an 8th floor balcony ...I'm missing two fish. They didn't jump out, birds and other critters aren't an issue. So if they died, would remaining koi eat them? There isn't a single trace of the missing two.
They will eat on the dead remains but usually you will find bits and pieces in there. Raccoons might be able to get up on a balcony and they will carry them off.