There is another couple of threads on this. Started on my 2012 the day after I got it (9k miles).
Was taking off the top and the horn stayed on like someone was laying on it. Took it right back to the dealer as it kept happening. They thought it was the alarm system but I knew that wasn't it right off the bat (kinda disheartening when you realize you have better sense about car issues than a service guy at a dealership)
Anyways did some research, found out others had this with different culprits. Water in the Horn, Radio, and cruise control buttons, etc seem to be the common ones so I shared my concern with the dealer on having to bring it back and forth while they played guessing games on what might be wrong.
They went the extra mile and replaced the whole unit (forgetting what they called it) but all the control module in the steering wheel instead of just the cruise switch or clockspring or whatever and I have not had the issue come back (knock on wood) for about 2 months now.