It's that simple. Even in thick fog, a burglar's hardly likely to park right in front of the house he's robbing. He's much more likely to drive 'round back and force the door that isn't as visible from the street.

The killer and his victim were unable to see the car and the forced door because they came in through the front door -- the intruders' car was parked out back, so they thought they were alone. Later, when the heroine goes searching for the house she remembers, she obviously looks at the houses from the front, which is why she can't find it again.

She not only fails to find it, but also fails to recognize the same house when she's standing inside it. When she returns to the house, she comes in through the front door -- so naturally the stairs are in a different position, and the layout of the ground floor seems completely different.