But wouldn't the dogs have found her scent before they reached the car park? The back of the apartment is across the road and not far from the lane.
When Rex reached Francisco GM, he suddenly was in a area swept by cars and wind etc., he nevertheless crossed the street, perhaps because he picked up the scent Madeleine left there many times (if they always crossed this street at more or less the same point). Then he didn't hesitate and turned right (showing he had really picked up something) but then, arriving at the lamp post the dog was lost. I think he was lost because the scent was getting weaker and weaker, indicating he was in the wrong direction. That's why he crossed where Madeleine had crossed at 5 when she came back from the creche. Unfortunately the secondary reception's door was closed and nobody opened it. So the dog lost the scent irremediably.
The fact the dogs couldn't choose the exit from the flat, the fact that nobody opened the door to the dog, makes me feel that nobody believed much in these dogs. A pity.