Ultrasonic sensor triggered

I was doing my online application for a forum today and then suddenly I got a call from my car. Yes, thats right. As I have installed an IES GSM alarm system onto my giant (Toyota Alphard), it will "call" me whenever any of the sensor in the alarm system is triggered. I quickly rush to the balcony and have a look. Well, no one was near the car so what's trigger it? The alarm also sent me an SMS stating "Ultrasonic Sensor trigger". So something has triggered the ultrasonic sensor.

Then I suddenly remembered I did wound down the windows this morning to "air" the car. The place is very windy now so the breeze has blown into the car and triggered the ultrosonic sensor. I did experimented with this feature before during the installation of the alarm system. The installer did demo'ed it to me by blowing air from the fan through the opened window and it triggered the ultrasonic sensor. Well imagine that even breezes can trigger the ultrasonic sensor, any movement in the car will definitely trigger the ultrasonic sensor. Now I am very confident that this alarm system can provide preventive warnings to me.

I went down to the car to wind up all the windows and now it is back to normal again.

PS. The ultrosonic sensor can be a nuisance at time especially when you are at the petrol station. The moment all the doors are closed the ultrosonic sensor will be activated, any movement from your family members waiting in the car will trigger the sensor and the alarm will be activated. The only way to avoid that is to leave the driver door unlocked, which is something I would try to avoid as much as possible.