Unfortunately we have gone through this as well since the car was new. It is quite random and hard to trace and very annoying. It is so frustrating to have no issues, start up and drive somewhere, shut the car off, 3 hours later come out to a dead 12v battery, knowing we have a traction pack with lots of power in it right there, but no way to access it.
We have resorted to carrying a 12v 9ah small sealed lead acid battery with alligator clips, it works like a charm, but a pain, even my wife knows what to do when the 12v is dead, so much so she doesn't even call me, sometime later she will just say "Oh I had to jump the Leaf again today". Also about a month ago I disconnected the shunt or sensor on the 12v battery itself, this causes the car to keep the 12v battery at 13.9v in my case, which charges the 12v battery more than it would otherwise. So far (knocking on wood) we have not had to jump it once and when I do a top off weekly charge it goes in to float in a matter of minutes where in the past sometimes it would be many hours.
I have tried checking phantom loads and only once have I caught a 2 amp load on when the car was "off". I checked everything and saw nothing on, I power cycled the car and then it was gone. I am convinced it isn't a small consistent phantom drain, but rather something larger that stays on and runs the aux battery flat. I so wish when I had caught it I started pulling fuses to see what it was.
We need a momentary emergency "push to jump" button from the traction battery