Tried another little experiment - keyfob only had to be about 5-6ft away from the driver's door for iDrive to come up with the 'No remote' warning. However... I was still able to shift from Park to D with the keyfob out of range.
If the engine cut out as the keyfob goes out of range, could be disastrous. Imagine - you drop off your partner who has the key in their pocket, pull out into busy traffic, engine cuts out...