Could be a number of reasons, is the alarm self powered? By that I mean does it have a built in battery which is designed to sound the alarm should your main van battery cable be cut by rascals. If it does then it is very likely that the battery due to the age now has a dodgy cell and could be triggering the alarm. Do you have ultrasonic detectors? Do you have swinging air fresheners or other silly items that hang in your van by string or chains? If so then the movement of these items can cause the alarm to sound as the movement is detected by the sensors and deemed to be intruders. Do you have a faulty main battery? If so sometimes the battery may have a dodgy cell which is more noticable in a colder temperature i.e. at night, the alarm perceives this as a volt-drop which in turn will sound the alarm.
You need to look into these possible scenarios first buddy.