Was transiting Madrid's T4 & after presenting my arriving flight's boarding pass upon request, the club receptionist asked for the departing flight's boarding pass. I explained that I was unable to get an on-line boarding pass from LH apparently because they want to physically see my passport before issuing the BP. LH is in T2 quite a distance away requiring use of the inter-terminal bus.
Receptionist went ahead and scanned my PP card & I signed. Thus gained club entry.

Since PP obviously pays the Madrid airport lounges for each entry, I don't understand which party would want to verify the boarding pass? Would presenting the trip itinerary be sufficient?