I think you are not wrong with the automatic light as the cause? If you cover the sensor when the ignition is on and the daylight is on, your driving lights should switch on immediately, and vice versa, they should also switch off again immediately. Can you also measure these voltage peaks at the battery at this moment?
I found an older thread here about an Opel that had similar problems.
motor-talk.de/lichtautomatik-entlädt-batterie
A special update of the relevant light control module brought the remedy.