There are a number of different things that can trigger the anti-theft system in your car. Here are some of the most common:
1. Leaving your car unlocked - This is probably the most obvious one. If you leave your car unlocked, it's easy for someone to just jump in and drive off with it. Even if you're just running into a store for a quick errand, it's always best to lock your car.
2. Parking in a high-crime area - If you live in or are visiting an area where there is a lot of crime, it's best to be extra cautious with your car. Park in well-lit areas and close to security cameras if possible.
3. Tampering with the ignition - If someone tries to hotwire your car or mess with the ignition in any way, this can trigger the anti-theft system.
4. Breaking a window - If someone smashes one of your car windows, this will usually set off the alarm as well.