I’ve googled, obviously I’ve looked in the manual, I’ve come up empty.
There is a round shiny disk on my motorbike, underneath the fuel tank, next to the tax disc holder. Seen from the front it’s blocked by the front wheel. I’d have though it was just a reflector or something similar, if it was visible and if it didn’t have wires coming from it. Any ideas?
Pictures:
Isn’t it just the horn?
It is the horn.
It’s a bomb! Run!
Yep, it’s the horn. Put your hand on it while you push the horn button, and you’ll feel it vibrating.
When you press the button it plays Rio.
Years ago the horn on one of my bikes stopped working. Or maybe it wasn’t loud enough. Something, anyway. I fixed it by tightening the bolt in the middle. Too loose or too tight, and it doesn’t work.
The contacts inside the horn get corroded. Operating the horn (and adjusting the stop) cleans them. We used to charge the shop minimum charge for this and called it a “horn tuneup.” 
We have a consensus. Thank ye muchly.
The first modification I recommend to every new MC owner is this: Replace that tiny disk with a real loud motherfarking horn from a car, or even better, a truck.
Black and white photos?