This is a question that is often asked and can be experienced at any time of day or night regardless to the burglar alarm being turned on or off. The reason this happens is because your burglar alarm system is constantly monitoring the status of all cables, detectors, keypads and sirens even if you have a wireless system regardless to the alarm being on or off. The alarm sounds to alert that there has been a change or has detected a fault with one or more devices on your system.
The most common fault is usually caused by human error! Make sure in any of the rooms that have detectors in them that there is nothing likely to move such as balloons, art works hanging from ceilings, wind chimes, plants and curtains that are moving from an open window etc.
Spider webs or insects on a detector can also be a problem causing your burglar alarm to go off intermittently. Other common problems can be caused by loose detector casings as they have sensitive tamper switches and cables that might be slightly damaged overtime causing the alarm readings to fluctuate.
Cleaning your detectors regularly using a damp cloth will minimise the chances of a false alarm. Opening or adjusting any burglar alarm system devices yourself will cause the alarm to go off, and a technical fault will appear on your keypad preventing you from using the alarm until it has been reset by an engineer. This is to ensure that know one tampers with the system and cannot be overridden.
If you have checked all the above and the problem still persists, contact your installer and book a maintenance visit as the problem will not go away on its own.