I think he's probably worked out that the odds are that it's empty (ie the inherent likelihood is that Rose's mother is indeed dead - the plane crash is a sufficiently public way of going for him to have taken a view on it) and, absent the Nicola complication, once he'd got her in the car if they drive up and it doesn't appear to be empty he can keep on driving as needed. He's an opportunistic type, clearly; see how he handles Nicola's initial suspicions.