I had this same problem with Octavia iV today. Yesterday the car worked just fine and I drove about 30 km using eMode only. I also charged the drive battery in the evening and unpluggeg the charging cable around 11pm when the battery was full. Today the car was completely dead and I had open driver's door using the mechanical key. 12V battery showed 4 volts. I was able to turn the car on when the tow truck driver hooked a full battery to + and - terminals. Car was towed to the dealership, let's see what they have say about the problem.
I've also had once a situation where radiator fan was spinning at full speed when the car parked and off. I was charging the drive battery when this happened and only way to make the fan stop was to unplug the charging cable. Outside temperature was around 20 C and the car had been parked for couple of hours after driving before I plugged in the charging cable. This has happened only once about 2 weeks ago. A guy at my local dearship said that this might be a same kind of software bug and told me to contact them again if it happens again.