Try arming the alarm but without the sensors. ( insert card and press start button 3 times , then lock car as normal( it says 5 in user book but it doesn't work with ours)
That way you can eliminate the sensors. If you still get false alarms then it isn't the sensors (ultrasonics) but a dorr/boot/bonnet switch fault.
As the car is still under warranty I would take it to your dealer anyway. You pay enough for a new car so you may as well make use of the warranty.