I ended up with the big C just mid December on a Friday. Got home after the PCR test and parked up. Didn't plug it in as I didn't see the point. Went to plug it in on the following Sunday (day of release), and the car wouldn't open either... I had forgotten to unplug the dashcam. It was recording 24/7 and the 12v battery had gone down to 3.1V.
Couldn't charge it on the 12v trickle charger as it only raised it to 3.6V after 3 hours. Decided to jump start the car to open the charge port (could do with a manual release for that!) then plugged the Pod Point in and left it overnight. Once connected it kept the 12v alive to charge the main battery to drive to work the next day, even if the 12v was flat I could jump it and get the 12v replaced.
However, to my surprise the next morning the car unlocked, opened the bonnet and checked the 12v and it had been revived to 13.1v. happy I didn't have to spend nearly £150-£200 to replace the battery.
TLDR Don't leave the car for long periods with anything plugged in to the 12v (OBD or Dashcams).
Also remember to lock the car as it could still be "awake" (not sure but when i walk past it my phone connects!)..