Random alarm on 06 Sebring Touring ragtop

A 2006 Sebring Touring convertible (70,000 miles) I inherited from the
original owner has a randomly occurring problem; the horn starts blowing at
random.
An ASE certified mechanic said it was the (factory) alarm system. He doesn't
work on alarm problems.
I have stopped it by beating on the horn button on the steering wheel, but the
problem recurs.
What's the best way to troubleshoot this? I am not healthy, nor am I
mechanically gifted.
I'd like to know if there's any way to gather more info before going to another
mechanic or a dealership.
Thank you in advance.