Many of the modules in BMW vehicles rely on the computer to tell them when to turn off via the CANBUS. If a module fails to either receive that message, or is defective, it could stay on when it should not be, and could be discharging the vehicle. Very few of the things in the car are controlled directly like a real switch...there's a little circuit card in there and a connection to the computer. FOr example, the light control switch for the headlights does not switch power to the vehicle lights! The connection to is has 3-4 very small wires in a miniature connector that in no way could switch the current required to control them; it sends a signal as to where it is, and the computer sends out another signal to tell the things to turn on/off, or switch modes. This allows the wiring to be much lighter, the connectors smaller, the switches to use low power contacts, and overall, saves weight and costs. My other BMW is the same way, so it is not unique to the i3. But, as I said, if a module is flaky (more likely a connection in the wiring is) it could prevent things from seeing that the car is actually off, and continue to try to talk on the bus, wasting power.