I can understand wanting to steal a koi... but doing it is pretty stupid. Any crook would do better stealing just about anything else. A dealer would risk their entire livelihood. An active hobbyist would soon be found out. The typical backyard pondkeeper would not pay value. Only a 'secret hobbyist' could ever get any benefit... and deserve not be able to share their possesion with others who might appreciate what they possess... a sort of self-imposed solitary confinement. I tend to think such thefts are more like senseless vandalism. The fish likely die in the process or get dumped in a public pond nearby. Too many senseless acts by deluded people.
Not many fish go missing, but do you think that there would be any advantage in having them "chipped?" I think perhaps the big breeders would find this useful--they could input I.D. numbers to hook up with buyers, names of parent fish, and birthdate. There might be other uses as well. Any ideas?