Today my Nissan car horn started sounding continually. I tried to turn it off with the plip thinking it was the alarm but it continued. Got into the car and started the engine up and then turned it off, still it continued. My neighbour was just about to disconnect the battery after about 5 minues and it stopped on its own. I took it to a garage and they ran diagnostics on it and no fault found. I am worried about setting the alarm now. Anybody know what it can be and what can be done?