Oh my this just scared me so much!!?

Usually house alarms are connected to a mains electricity supply with battery back up. If the alarm has been disconnected from the mains supply for a while ( during a power cut for example) then the battery back up kicks in to maintain cover. So if this alarm has been off from mains supply then the battery back up will have cut in, but when that too gets low it will cause the alarm to activate. I would suggest that you get it back on mains supply again otherwise this will continue to happen at all sorts of inconvenient times ... day or night