I've had a problem on and off for about 3-4 months with the alarm on my astra.
After locking the car, normally about 20-40minutes later the siren, horn and indicators all go off. I thought it was the power sounder batteries at first but after doing some reading it appears only the siren goes off if thats the case?
This is happening every time I lock the car now (I think... I don't hang around long enough when I'm at the train station!)
Once I unlock and then re-lock the car though, it very often doesn't occur again until I drive it and park it up....
Any ideas, it's driving me mad!!