Nah, mate. It's a feature on the high-spec models to help the hard of hearing...
Your suspicion of a shorted wire is quite likely, but also remember that if the horn has a bad ground, that could also make the current find an easier path back to the battery through the sidelight circuit.
Pulling the fuse for the sidelights may help to narrow down the problem, if the horn stops working or affects a different circuit.