The handler knows his dog and the dog's body language. He behaved differently when he approached that car, just as he behaved differently at the door to G5A. That tells the handler the dog could be in scent, so the handler then directs the dog to search more closely.
How was Eddie's behaviour 'different' when he inspected the Renault? IIRC he circled the car and then moved off - in exactly the same way he did with the other cars. He showed no more interest in the Renault than he did in any other car - until he was repeatedly called back and made to 'concentrate' on the Renault.
My argument is that if Eddie, with his ability to detect minute traces of alertable material, had been required by his handler to give the same amount of attention to the other cars, then alerts may well have resulted from one or more of those vehicles.
IMO it was Martin Grime's behaviour (not Eddie's) which changed when he reached the one car which was clearly identifiable as belonging to the McCanns.