The Radio Code Card is supplied with the car from the factory and is the grey folded card in 306maxi's picture.
We leave them in the owner's pack for the customer (to lose) and record the radio code on the PDI sheet and under the vehicle details on our system for future reference.
TBH, unless you're swapping the radio from one car to another, it shouldn't lose it's code.
What about flat battery/battery removed?