Car alarms usually go off when a heavy object hits your vehicle or when someone attempts to break into your car. If it suddenly goes off for no reason at all, it may be caused by a faulty key fob or a low car battery. Issues with your key fob can send wrong signals to your vehicle and create a false alarm. It can also go off if you try to start your engine with a rusty or dead battery. If your alarm still keeps beeping despite a full battery and a working key fob, then you may need to get it checked by an expert.