Accenta G3 goes off when un-set?

Our Accenta G3 has gone off three times, always in the early hours of the morning, whilst being un-set. The panel shows no tamper or other alert lights, in fact it looks entirely normal. If I ease one of the sensor covers open the tamper light shows, so it is working OK, also if I force an alarm for any of the door or window sensors the panel shows them accordingly. I have been unable to access the event memory for some reason. I would very much appreciate any suggestions!