Here's a fairly complete list:
4.0 Boxster Configuration - Porsche Boxster (986) FAQ
I believe M535 might be it: "Remote Controlled Alarm System". From the site's details of this code:
"-A remote entry device. In the base of two of the ignition keys for the Boxster are two buttons that you can push. One button unlocks the driver's and passenger's doors. The other opens the trunk.
-The alarm will sound if the driver's and passenger's doors are opened, front and rear trunk are opened, oddments compartment is opened, the flap that lifts to store the cabriolet top is lifted, radio is removed, ignition is activated, key with invalid transponder code is used in the ignition lock, horn is disassembled, passenger compartment sensor is disassembled, voltage supply to the control module is interrupted.
-Interior infra-red sensors that are active when the hard or cabriolet top is closed. Near the reading lamp and rear view mirror are two infra-red sensors that detect whether someone has reached into the passenger compartment when the car is locked. For example, if the cabriolet top was cut and someone reached in, the alarm would sound. When the cabriolet top is open, these sensors are disabled. However, you can get a clip that attaches to the handle between the two sensors that reactivates it even with the top down. The part number is 986.504.986.00 and costs less than $2.00. You can also simply push on the microswitch where the hook for the cabriolet top connects for a few seconds as you activate the alarm.
-The alarm sounds by honking the car's horn for two minutes.
-There is no annoying beep as the alarm is activated and deactivated. Feedback is shown by blinking the front and rear parking lights twice to show the alarm is armed, once to show it is disarmed. The door lock switch and another red LED in the center of the dash also blinks.
(Note from O/D: This is not true. Along with the above described activity, the horn should give a very short, single beep when the car has been locked, unless you double tap the lock button, which will prevent the motion sensor from activating, accompanied by a long flash of the signal lights and no horn beep.)
-If the car is not secure when the alarm is activated, a single short honk of the horn will notify you. The most frequent cause of this honk is because the interior oddments compartment is not fully closed.
(Note from O/D: Again, this is not quite correct. The horn will sound differently from when it normally locks to let you know that it cannot properly lock, and it will do this as soon as you press the button.)
-If you unlock the car or trunk and do not open the driver's or passenger's doors within two minutes, the horn will give a single short honk, and the car will relock. If some random stray RFI signal managed to unlock the car, it would only be unsecure for two minutes.
-If you do not order the alarm, there is no way to protect the contents of your front and rear trunks when the cabriolet top is down. Anyone can reach in and open the car from the inside by pressing the unlock switch on the center console, then open the door and pull the trunk release levers. If you have the alarm, this will sound the alarm and the trunks remain locked.
-If the car is not opened for about three days
(Note from O/D: 5 days), the remote entry on the key will no longer work. You will need to use the key in the door lock to unlock the car.
-All cars (with and without the M535) have a device that prevents the car from being started without a microchip in the car key being present when the ignition is turned.
-The M535 option does not include motion or proximity detectors. You will need aftermarket equipment for that."
See if that code (might be lacking the M) is on a label on the inside of your front trunk lid. It's usually a white label that is toward the top center of the lid (as you look at it lifted, standing in front of the car), and will have a bunch of 3 digit or 3 character alphanumeric codes on it.
I believe this next one is the most complete list one can find at the moment. It calls the M535 code "Anti Theft Lock 315MHz".
986online.com- Boxster Option Codes -Porsche Boxster Parts And Accessories
So while you're at it, might as well check all your codes and ensure that you do indeed have everything that's on the label.