That does seem like a quite strange approach. If some people would approch me at the airport and ask me for my CPR number I would be more than suspicious (except if they'd be police of course) - besides the legal point of view. Someone brought up the ticket inspectors on Swedish public transport, which I think is a different case: You are required to show a ticket when you use public transport, so if you don't have one you are clearly doing something wrong. But you are not required to give random people at the airport your CPR number - or am I wrong with that?