I use something called an amp hound ..
https://www.amazon.co.uk/Amp-Hound-Voltage-Drain-Tester/dp/B00M7S28MQ
Not the cheapest tool around but it saves haveing to pull fuses .
Do a discharge check and post the values you get .. on a 996 you will have to wait at least 30 mins and probably 60 mins to get an accurate reading ..
996 do a lambda sensor burn off for approx 30 mins so the readings will be high untill it stops .
A pulsed discharge of approx 1 amp could indicate a tracker has been fitted and the battery has failed.
Very high discharge of say 4-6 amps and its seat adjustment motors.
I expect you will have 1 - 1.5 amp discharge and i would start with the PCM , amp etc ..
Remove and unplug the PCM to see if it drops .
Alternator .. hmm .. ive seen it once on a Cayenne , discharge of 1 amp .. very rare if its that .
Have a quick check under the passenger seat .. make sure its not damp or water bottles stuck there that might be leaking .
If you pull fuses then DONT pull any alarm ones .. dead meter and painfull ears at that point !
To test i open the drivers door and drop the catch so its locked , ignition on and connect your meter , then lock with a double press of the key to turn off interior monitoring .
Or just drop the door lock and as above but dont lock and wait till it goes to sleep .
I prefer the locked way as thats how the drain would normally be but either way will work and less chance of an alarm trigger if you dont lock it .
Alarm trigger tends to blow a standard meter as its 10 amp rated .. ok if you have a higher or surge protected meter though.