Because the lounge would technically be in A/Schengen, to enter/exit from/to Z you have to show your passport at the entrance. But to exit to B by car you would have to leave Schengen again by the elevator to the platform. Technically, this could be performed at the exit to Z but I have no doubt that the BPol would want to see who is leaving through the back door elevator to B/exit Schengen In MUC the car transfers are done through the same entrance/exit.