Alarm going off when the car is not locked

I've just bought a 996.2 and I hope I am making some mistake with the alarm, but I fear something is wrong with it. If I sit in the car while it is standing, for instance making a phone call, the alarm can suddenly start beeping. This definitely occurred with the key in the ignition. I don't remember if it occurred with the key out of the ignition. Also, on several (all?) occasions, opening the passenger door from the outside set off the alarm (when I did this, the key was not in the ignition). And it seems to go off in some other situations as well, when I am in or near the car. In case this might be related, I should mention that only the main button on the key works. The two smaller buttons seem to do nothing. Any ideas what could cause this and/or how to fix it? Thanks!

Quotes source post: 11681990

Reads like an ignition switch, but I would advise you to before having this replaced to get a professional tech's opinion. In the meantime, you might try wiggling the key about or pushing it -- gently -- one way or the other to see if you can affect the car's alarm behavior. While you there check the car's cabin floors for signs of water/dampness. Check under the passenger seat in particular for that is where the alarm module resides. Be careful. There can be some sharp edges under the seat. Check the body water drains looking to see if they might have been allowing water to collect and possibly overflow into the cabin. If you remain in the car but *without* using the phone do you notice any unusual behavior? I'm thinking maybe the cell phone is partially to blame? There's a reason these are forbidden to use on air planes. If I put my cell phone in the cubby hole in the center stick of the dash I sometimes hear the phone acting up. No alarm issues though.

I used the car today for the fist time since writing my original post. I opened and closed the frunk and the passenger door a few times with no issues. But then I opened the driver door and the alarm went off. This time definitely without the key in the ignition. I seem to remember from going through other treads about alarm issues that the alarm sounds like the horn beeping. Mine sounds like that plus a siren sound. Could that mean I have two alarms? Or does this combination of sounds mean anything?

Quotes source post: 11684343

And Macster, thanks very much for the helpful post. Regarding the ignition switch, could this be the issue even if it happens when there is no key in the ignition? And if so, could you clarify what you mean by this? (Otherwise, I'm pretty sure the floor is dry. I do know the seller replaced the floor mats, not sure why. (No mention of replacing the carpets, which are in very good shape.) So it could be there once was a moisture issue ... .)

Quotes source post: 11684595

The factory horn alarm is under the driver windshield cowl and is easy to disconnect if you want to test out where the other sound is coming from.

Quotes source post: 11684595

Could have a 2nd alarm. I have read that some insurance companies offer a discount on insurance (theft insurance) if the owner fits a certain type of alarm. Believe it or not often this comes up when the new owner experiences a problem with the second alarm and learns for the first time the car came with 2 alarms. The door opening could be some switch/sensor in the door acting up. Or it could be a sign the door has water in it. Give the doors a shake and listen for water sloshing. If there is a door switch problem using a Porsche diagnostics computer might turn up something. When I had my Boxster in for weird alarm problems that were ultimately traced to a bad door lock assembly the tech told me error codes obtained from the alarm module pointed at this assembly. Oh, what I mean is the switch contacts either do make contact when they should or do not break contact when they should. This results in very weird behavior not always just an engine that won't crank or start. Often dash lights act up or the headlights don't work right, stuff like this. It could still be an ignition switch problem but the odds are getting longer on this. New floor mats could just mean the seller wanted to improve the looks of the car's interior before selling the car. Floor mats can get really ratty looking and it is a cheap cosmetic improvement. It could also arise from the mats being ruined -- stained or developing a foul odor -- from having gotten wet over time. You may be faced with having to remove the passenger seat and carefully checking the alarm module and surrounding hardware to signs of water damage. There is another possible explanation. Again a long shot but not out of the question. The car is new to you and it could have had a rodent problem, suffered rodent damage. How much of a check out of the car did you do before you bought it? A tech can inspect the car and spot rodent damage sign. Techs see this more often than you might think.

Update: My car is a Targa, and I learned from my local specialist that with Targas and Cabrios, the alarm arms itself after a period of inactivity; the idea is apparently to sound the alarm if anyone reaches in or enters through the top. If this is true (strange that I've seen no mention of this on the forum), then it would explain all but one of the incidents. But he thinks the alarms should not alarm itself if I have not left the car - the car should know that the seat is occupied - so the situation where it went off while I was making a phone call is still unexplained. Also, in the original post I wondered if this might have anything to do with the small buttons on the key not working, but it turns out they do work - I just needed to press them longer.

Quotes source post: 11687421

If this is the case this feature should be covered in the owners manual. I'd have to have something more than just a specialist this feature is present and works the way you were told. If this feature is present then and arms itself after a time with you still in the car this suggests to me the seat switch that signals it is occupied is not working.