Ghosts headcannon: boundaries are actually constantly changing. If you die in one area, and that area has something built on it later, then boundaries change. Depending on where a ghost is when the boundaries change determine their new boundaries.
Robin probably had either a much wider/smaller area to roam before the house was built. When the house was built, he was in the new boundaries of the house. If he was outside of the boundaries of the house before he died, then I don’t think he’d be on the property.
The cholera ghosts stay where their village died. Assuming their village was bigger than the house and its boundary, there’s probably a few cholera ghosts that are trapped outside the boundary because they weren’t there when the new house boundary came in.
If a car ghost had their car crushed, then either they would be trapped in the scrap yard where their car ended up, and then become fixed to the boundaries of that yard, or they become a free roaming ghost.
