In most cases this will due to your Backup battery in the Intruder Alarm panel no longer holding enough charge to power the alarm system.
In a professional Intruder Alarm system, there should be two rechargeable batteries. One in the Intruder Alarm Control Panel and another in the external siren (Bell).
Control Panel
The control backup battery is suppose to retain power to the alarm system in the event of an electrical mains failure, normally the battery will take over and all that you will notice is a fault appear and your system will continue to operate as normal.
If the battery can no longer provide enough power to the system it will go into alarm, this is a tell tell sign that the battery needs replacing. During routine serving of an alarm system the battery is tested and will be recommended for renewal should it fail. The lifespan of the battery is around 3 - 4 years.
External Siren
The battery inside the external siren will trigger the sounder if there is a system power failure whether the system is set or not. This is what the alarm is designed to do and will sound for its programmed period of time. The lifespan of the battery is in excess of 10 years, If this has failed a new Siren would be recommended.