I've had the same problem with mine, and it is caused by another key on the
ring dropping between the steering wheel and steering column when you insert
the keys.  The key tip shorts the horn wiring, allowing the beep.  I'll bet
if you use the key off the keyring by itself, it will not do that.
Steve Sandlin
Steve's Lock Shoppe   http://www.steveslockshop.com
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle