When I walked to my Jazz, it had already gone silent, so I took a look at the fusebox. Sure enough, it had blown a fuse due to the prolonged horn blast. My steering wheel's horn pad had developed the habit of getting stuck. Strangely, it had to happen when I wasn't even in the car!
The blown 15A fuse protected two circuits: the horn and brake lights. With new fuses at hand, I tried the horn again. A faint but ominous click was audible, but the horn kept silent. Driving around with no brake lights is pretty dangerous, so I put in a new fuse and resolved not to use the horn for the remainder of the week.
The long and short of this incident is, my quest to customize my Jazz might have taken another unexpected step. The short-circuited stock horn wasn't much good at making my presence known, anyway. This might be a good chance to get a set of more powerful air horns.
If only those new horns don't short from the horn pad getting stuck again...