What we all must be careful of is car theifs here in the UK are holding transmitter/receiver amplifiers up to front doors of houses and bridging the gap from key to car and gaining access to the car, then driving the car away.
The immobiliser chip is also read at this stage and once the car is started it can be driven away without the key until switched off.
I tried this with my i3 and left the key with my friend drove for an hour and the car did not cut off.
Maybe BMW should build into the software a constant one minute re scan of the key to combat this.