Credit Card Used for Booking
I seem to recall that the self-service check-in consoles scattered around the T5 concourse ask one to insert the credit card used for the booking... presumably as a quick way of identifying your booking so that one can check-in... not sure what happens if one has a number of bookings on the same card... maybe it gives one the option to select where one wants to go... or maybe it's clever enough to give only the option of checking-in for a flight on the day one is interrogating the machine.