There are a couple of threads on this. I also have a 13 S and have the same problem. It is just so random, my guess is something is sticking or staying on for some random reason and runs the 12v dead. The funny thing is the Nissan tech said that can't happen becasue if the car sees the 12v gets low it will automatically start itself and charge the 12v battery from the HV pack. Well.... that doesn't happen. I would believe that the car may sense a low 12v battery condition and then "try" to close the HV contactor to start charging the 12v, but that is also run from the 12v battery and guess what, it's to late and it's already almost dead and trying to close that big contactor just runs it totally flat
I so wish I knew what it was, I weekly charge ours now no matter what, but just last Sunday it was dead again, and took 35 amps to get the 12v back to "full".
And before any asks, no I don't leave my leafspy plugged in, I only plug it in when I am actively using it and unplug it afterwards.
This is my biggest pet peeve with this car, and it has happened to my wife out shopping and once at her parents. I keep a 12v 9Ah lead acid in the car all the time now to "jump" it. She had to call me and I had to walk her through how to jump it. She is not happy about that.