When you have switched the jumper pin for the battery over, remove the 12v supply to the bell, if it rings then you do not need to replace the battery.
It means u will need the engineer code to reset the system after a tamper alarm of its on. I was thinking more along the lines of a loose wire being disturbed as u oress the keypad..... have u opened it yet ?
Could just be a loose wire lol u wont know til u open it. U can silence the tamper alarm with fob once activated....... hopefully not been left on engineer restore.......