Even with the alarm drawing power, the radar sensor, the ECU drawing power, for a battery with 800amps of available current when fully charged to be discharged enough after a week or two to a point of not being able to start the car indicates a problem.
The electronics do not draw that much current when the car is off.
The problem is related to the rest of the electrical system most likely the wiring...maybe a bad battery cable, starter cable, bad relays, something.

 

I also had an issue of low voltage in a year-old AGM battery in my '08 997.1. I read somewhere (maybe Pelican?) about hard wiring a small Battery Tender directly to the battery, and plugging the thing in when I left the car for awhile. The little red light eventually turned green, indicating it did its job.

But after driving the car one day this week, making three local stops for errands, it barely turned over and I was lucky to get home.

I understand the comment about needing to drive 25 miles a day to keep it charged, but the Battery Tender had been plugged in until that morning, so I assumed I started out with a full battery.

My electrical engineer neighbor came over with some test equipment, and we went through everything. Starting the engine, the alternator and voltage regulator were working fine, 14+ volts, and when I turned off the engine, the measured parasitic load went down after a few minutes to a very small amount.

It seemed like we had a real mystery here.

But then we checked the output of the Battery Tender, and found it to be zero. The charger was no longer working despite its little red/green indicator light.

I have since used a full-size automotive battery charger to bring the battery up to full voltage, and plugged in the Porsche portable charger/maintainer that goes into the always-hot DC outlet on the side of the console overnight.

Problem fixed and mystery solved.