You need to be more specific about your car. What year is it? I see you've posted in both the MKII and MKIII sections, which is it?
In the MKIII FAQ section there's a thread on how to remove the MKIII's security system.
The factory alarm systems use switches which are mounted to the back of the three exterior lock cylinders. They wear out over time and eventually cause erratic arming/disarming when locking and unlocking with the key. Typically the driver's side switch dies first but unlocking with either the passenger door or hatch will disarm the alarm.
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