Hi , Matha, I seem to remember having the same problem on a previous car, after garage investigation they found a chip in the alarm system faulty, cost an hours labour and a new alarm, I will not frighten for with the cost. It also could be one of the sensors, these also are not cheap. It happened again so we ripped the ruddy thing out, The siren that is as nobody took any notice anyway. Sorry to be pessimistic but you will really need an electronics engineer to inspect.