I had this same issue when I took my truck out for a tour on Christmas Day. I thought that I hit the horn button when I pulled out of the driveway, horn blew when I went around the next corner and I was sure I never hit the horn button. I continued on tour after unplugging the horn relay on the firewall. I got home, took off the steering wheel and found the Horn Contact Spring/Pin sleeve was broken at the bottom, letting the copper pin contact the horn plate at will and completing the circuit for the horn to blow!!! I bought a new horn contact spring pin and sleeve assembly from Classic Muscle out of Winston Salem. I had to make the sleeve fit the hole in the steering wheel, (sleeve was too large dia by .030"). It was plastic so it was easy to change to fit. After re-assembly this worked great, horn blows on demand now!!!

1958 Chevrolet 3600 NAPCO 4x4 Apache
"Calvin"
In the Stovebolt Gallery