An Autopage car alarm can be set to an auto arm alarm setting also referred to as passive arming. When the alarm is in this mode, it will arm 30 seconds after the last door closes. It works as follows: After the ignition is in the off position, the alarm will flash fast for 30 seconds. After 30 seconds have passed, the alarm will automatically arm. The siren will chirp once and the parking lights will flash. If any doors are opened before the 30 seconds elapse the alarm will not arm until all doors have been closed again for 30 seconds.
For more info, see How to Install an Autopage Car Alarm