Battery empty, but not defective – what can it be?

Hello, yesterday morning the battery of my V50 was completely unloaded (residual voltage according to the ADAC measurement: 3 volts). Tags before I drove 150 km, and all consumers are switched off when the car is shut off anyway. There was also nothing in the socket of the luggage room, and the heater had not been in operation either. The Volvo workshop checked the battery, as well as the alternator – all supposedly in order. Of course, the battery is now fully charged again. And this morning the Volvo jumped perfectly. The workshop advised me to leave the car there for a few days to go on troubleshooting. Probably I will do this (of course I have to clarify with Volvo Assistance beforehand because of warranty, in order not to remain surprisingly at the expense). Has any of you ever had a similar experience or an idea what might have been going on there? My V50 D3 is now almost 12 months old, has 24500 km down and in puncto power/battery/starting had never had any problems. Greetings Thomas