Today, I've found my 12V battery dead again. This is the third time this year. Last time it was about 2-3 months ago. Since then I've been very carefully to watch the voltage reported by LeafSpy every time after stopping the car. But yesterday, before arriving home, I made a short break and I turned off the car in the same time while stopping LeafSpy on my phone. Then, after 5 minutes, I drove home(few streets away) and I didn't start LeafSpy again for such a short trip.
In the morning the 12V battery was dead. I had to use the mechanical key to open the door. The interesting part is, the "Link" LED on the ODB Bluetooth device I have, was blinking continuously at a pretty fast steady rate. My first though was that somebody connected to my ODB device and that was the reason why the battery was drained. But I don't think it is the case, because as far as I remember, you can pair with the ODB only at power on. So from now on I will try to check the "Link" LED to see if it is linked to the battery issue, or it was just generated because the ODB adapter had not enough power to work properly.