Zone keeps setting alarm off?

Hi when we go to bed and omit the upstairs after about half an hour the alarm goes off, it says 2 on the panel which is one of the rooms downstairs, we have checked all rooms downstairs when this has happened but nothing has moved ? We have renewed the battery as this was happening before we changed the battery? Any ideas please, it doesn't happen when the whole alarm is set? Do the sensors wear out? Is there another battery in the box on the wall the small panel one? Do I need a new control small box? Thanks