As it went off when you switched it off and unplugged battery, shows that the battery in the outside unit is working, Did it sound when you switched the mains off, with battery connected, if not battery ok. As it shows 24hr alarm, that is the tamper cct, So check all the units door sensors etc you must have a bad connection somewhere, Could be a cover on a movement detector, or corroded connections on a door unit, if any water has got in with the bad weather. If all else fails, disconnect battery and mains supply again, the battery in the outside unit will then sound off the unit, but will run the battery down, which will stop it completely, until you get it sorted.
SD
July 2012