Great observation about the gas door being open. Earlier I speculated that Wint might have siphoned gas from one of the family cars, or forced SS to do this.
Are you sure about the Land Rover being spotted parked on the street? Where has it been since that time, if it's only being towed away now?
I think the car you're thinking about is a Ford, shown in this distant shot in front of the police van:
This may be a detail shot of the same vehicle:
The first shot shows the white Porsche SUV and red Audi parked in the driveway. This means that the Mosler and the Land Rover could have been the cars parked inside the garage that day. W-1 may have had the Bentley. And SS may have come home in the Porsche 911.
If the Land Rover were in the garage, it would be a logical candidate for use in siphoning gas. The door would be left open in case Wint decided he wanted to siphon more gas.