alarm sounding on my 2000 zafira?

my wife has been clomplaining of an alarm sounding every now and again when she is driving the car it only lasts for a couple of minutes and no lights go off it is just an audiable alarm and today i went outside and walked past the car and an alarm was going off inside the car but again it only lasted for a couple of minutes. any ideas would be great as i dont have a clue.