Theres 2 parts to the equation ,a plastic holder that is in the glove box and a plastic key to let you into the car.once you've let yourself into the car you need to remove the holder from the glove box insert the plastic key into the holder and then insert into the slot on the dashboard and push to start the car.I recently bought my used S5 from an Audi main dealer and on collection the plastic key was missing the salesman didn't even know that there was a holder in the glovebox.To cut a long story short they couldn't find the plastic key so they had to order one from Germany,post it to me and pay for me to take my car into my local dealer to have all my keys recoded to the car.If you check in your cars DIS it tells you how many keys are registered to the car.I only knew about the plastic spare because i'd bought a brand new S4 previously which obviously came with a full set of keys.Nedless to say my glovebox doesn't contain my spare keys.