Anti-hijack mode should only kick in as Kato has said if the Drivers seat belt has been undone, or even if the seat belt is done up and the drivers door is opened then shut again the anti hijack will start its chirping for 40 sec.

I'm not sure if this works on all models but if you start your car with your seat belt and door open then jump in and buckle up and close the door the anti hijack wont kick in if you open the door or take off your seat belt.

Have tried this in my car and it seems to work.