If the battery in the fob is too weak for the vehicle to detect it OR there is too much interference (I've run into both situations, but rarely), you must use the actual key to open the door. If it was not interference, and the battery is too low, you must hold the fob up next to the steering column in a quite specific place and it's internal RFID can be read and it will start the car. I think BMW uses a multiple path authentication, and may not be subject to a simple repeater. Brute force computing, maybe.
It's my experience that if the car is running and you remove the key fob, it will continue to run (on my ICE), but you can't switch it into gear. On the i3, if you open the driver's door and remove the key, it goes out of ready state and into park. If it thought there were two fobs in the car and one was removed, (I've not tried this), it may just continue. If the fob's presence was because of a repeater, eventually, you'd get out of range. Once you stopped, I think you'll find it won't start again. Maybe useful if you wanted to strip it, but not much use if you wanted to continue to drive it away.