The car horn suddenly started sounding continuosly (not an intermittent beep-beep) as if someone was leaning on the steering wheel horn button. There were no flashing lights but a bleeping warning bell could just about be heard in the cabin.
When the key was put into the ignition and ignition turned on (but engine not started) , the horn and bell stopped. When engine was turned on, they stayed stopped. When engine stopped and key removed, both horn and bell started up again.

Any ideas what's wrong ?

I haven't looked in the fuse box in the passenger footwell.

Could there be something wrong/dislodged in there ?