I had a similar experience
I had a similar experience when I first got my Hybrid. I parked it in my driveway overnight thinking that I had shut it off but when I returned to it in the morning I discovered that I had left it on all night. I'm guessing that it simply cycled through using the battery until it ran down and then turning on the engine to charge the battery, over and over.
I always use the button on
I always use the button on th door handle to lock the car - 2 beeps. If you don't get 2 beeps, you either forgot to put the car in park position on the gear shift, left the key inside the car or you did not turn off the car, and will hear a series of beeps that warns you that sonething is wrong. The Altima is my favorite car of all the cars I've ever owned over the last 40 years. (Expedition, Diamonte, 280ZX, Cad., and others)