Found it.
Use of Horns
27001. (a) The driver of a motor vehicle when reasonably necessary to insure safe operation shall give audible warning with his horn.
(b) The horn shall not otherwise be used, except as a theft alarm system which operates as specified in Article 13 (commencing with Section 28085) of this chapter.
Amended Ch. 993, Stats. 1977. Effective January 1, 1978.
Seems rather vague to me. You were using it to ensure the safe operation of your vehicle. Fight the hell out of it.
THat is by far the stupidest ticket I have ever heard of.
Ever.
Plus in Code #27000 they require you have a horn in good working order - This is the stupidest ticket of all time.