[JLRnumber5]

If I remove my phone from the Lightning/USB lead when I stop the engine and lock the car, but leave the lead plugged into the connectivity socket in the storage box, does this make a small but continuous drain on the battery? If so, should I remove the lead everytime I leave the car for any period of time?

If there's nothing on the end of the lead drawing power then I think you've answered your own question! Maybe the cable has an LED on it but I'm sure the battery could power that for a few months ;-)