Very efficient system - you park and at reception you drop key into a machine which gives you a green receipt (with printed code). Whilst you're away, they move the car to a separate area and attach a plastic tag to your key which matches code on your receipt. All keys are kept securely at their reception. On return you hand in green receipt for key. I was very concerned about the enormous number of yellow plastic tags that must be used every day and then discarded. I wonder if at the exit, there was some way that drivers could remove the clip and drop it into a container ready for reuse.