Horn problems
 

Hi everyone. 1st time on. Have the 2005 convertible Mini Cooper which we leave covered at our caravan storage compound. Every week we travel down there to give it a run out. This time, took it for a run. Left it for about 20 mins, and the horn started of its own accord. Pressed the horn in, and it stopped. Once again, it went off after a bit, and continued to do so. Only stopping for a short while after I had pressed it in. Had to remove battery cable in the end, as couldn't risk it going off whilst I wasn't there.
Any answer to why it should do this.....