Hi and Welcome
It sounds like you need to get someone to look at it, I would recommend an alarm company rather than an electrician.
If you dont use it you should get them to decommission it, alarms are designed in such a way that they are difficult to circumvent, and when they are not serviced this can have side effects like the one you experienced last night.
The external sounder has a battery and can sound regardless of any connection to the control panel. The panel also has a battery with a 4 year life expectancy, if this has failed it will start to pull on the charging circuit, in some extreme circumstances could cause a fire. My advice would be to search for a local company and get a few quotes to decommission it or to recomission it and provide a maintenance contract.