If you can VCDS it, there is a measuring block to tell you what the last four activations were.
Talking from experience of changing the siren a few times on my mk5's, the problem is usually...
Siren itself is failing (internal battery has died or burst).
The battery is packing in.
Faulty ultrasonic sensor inside the car.
Less likely is a fault in the CCM or BCM in later cars, or wiring problem.
Is your car parked in such a way that someone going passed can create a bit of turbulence to rock the parked car? Happened once or twice to my Tiguan when boxed in next to walls, and it is windy.