My Dad had all his koi mysteriously disappear too and they were also large. Usually when he goes away for a spell he comes back to find one or two gone or injured (which then die). He had a problem with racoons for a while and installed netting and chicken wire barriers all around the pond. The last trip he came back to find all koi gone, and all talapia still there! No fish heads or bones anywhere in sight, so ruling out racoon. Netting was still intact so ruling out herons. In his case I think some neighbors who knew he was out may have taken them...as bad as a thought that might be, nothing else makes any sense...