[Overdrive]

Not sure if you have the manual, but you may want to have a look in there if you do. I believe it mentions that it is possible for a Porsche dealer to modify the security system and its functions to your preferences. Could be the previous owner had them modify it. Another possibility is that the car didn't come with a remote key, which wouldn't prevent it from having an operating alarm system, it would just be controlled by you manually locking and unlocking the car.

When you lock the car, does the red light on the upper center of the dash flash a few times every few seconds?