I had this happen today. Drove 10-15 minutes to a store, shopped for 10-15 minutes, then upon starting I had very little brakes. Pedal went nearly to the floor, and when the brakes did finally engage, it was abrupt and noisy (clicking/whirring/bumping sounds). Battery voltage seemed okay (12V-13V resting, 14V+ under charge). Plus I have a LiFePO4 battery, which is supposed to help with the Leaf 'undercharging' problem. It was somewhat cold: we had a bit of snow last night, and just before that a lot of rain, and on a short drive during heavy rain, I drove through some large puddles, creating lots of spray. I just mention that as something could have gotten wet? But then I would expect the failure to not wait until the next day.
I had the car towed to a nearby dealer. Having read this thread since then, I may have been able to get it home and try a few things. I suppose if the LiFePO4 battery is failing, that could explain it, but as mentioned, voltages look good (on a cigarette lighter voltmeter). I get that a 12V battery load test is more telling of battery failure, but I wasn't in a position to do that test. The dealer might do that; we'll see what they say tomorrow. I have been getting intermittent Air Bag dash icons and DTCs (B00A0 0008 AIR BAG is still showing above), and those have also been blamed on 12V battery problems, so maybe that's it. And that seemed to start a few weeks back when it was very cold for a couple of days (below freezing).