Probably there is a short in the wiring somewhere. But there are two other possibilities. 1. There are several power relays that turn off when the key is turned off, and if one of them has a short in the wiring this might cause the horn to honk. It could have just as easily turned on the dome light, the starter etc. I just happened to be the horn. 2. The way cars are wired now is that some wires are used for multiple things by using diodes. If a diode has shorted then it is not resisting the current anymore and could allow current to flow when the key is turned off. If I were starting to look for the answer, I would pull the ignition switch and check inside the steering col. to see if any of the wires there are shorting against the ignition switch connections.