Cold temps alone shouldn't result in a state of charge decrease. The inverse would be that the battery could be charged to 100% when cold, then rise to greater than 100% when brought back to room temperature, which we know isn't the case.
The low temps mean the battery won't operate as efficiently, ie fewer miles per kilowatt. But 100% SOC is 100%, and 50% is 50%.
The methods the car uses to estimate state of charge could be playing in to what you're seeing. The displayed SOC isn't an absolute, but a very "educated guess" and is updated after the battery is at rest for a while. In other words, it's possible the SOC was recalculated to a more precise estimate.
Perhaps the 12v was getting a top-off at the end of the drive, as well?
In regards to the i3 being "left on," I'm doubtful. If I step out with the car powered up and radio playing, everything shuts down once I close and lock the doors.
The exception I believe is if I have a destination set in nav, but have stopped off before reaching it. I believe the climate system stays active just as if I had selected Activate Comfort Climate Control from the shutdown menu.
