Even if you left your car plugged into a wall or mobile connector, the HV battery will lose some charge over time. Especially if you use the app to check the status of the car. There's several hundred watts of power used up when the vehicle is "awake" and it will continue to consume power until it goes to "sleep" after some given amount of time (minutes, not hours). Assuming that you do not have Sentry Mode or Cabin Overheat protection enabled, there's very little power used. But when you check in with your app to see what the battery level is, or play with opening and closing the trunk, it wakes up again and start draining some more power.
After some loss of battery power, and assuming your car is plugged in, it will start to charge, up to the last limit you've set. I don't charge at home myself but I believe a loss of 5-10% will be enough to trigger the start of a charging session.