The ghost cars are situated
in Back O' Beyond, and is really nothing more but a couple of
beaten-up Glendale cars. There are two different Glendales in
the game, one ordinary Glendale, and one with dents and what
seems like chainsaw marks (which has been explained as a hint
to that Leatherface actually exists in the game). The two ghost
cars spawns in the hills in Back O' Beyond, and are therefore
rolling down the hills as you spot them. They're not driving
by themselves, just rolling down, then stopping. Nothing more
weird to that.
A pedestrian is also heard saying this: "My car move by itself when i go shopping.". I don't know if it's related to the ghost cars, or just a fun quote.

Somebody also made this arrangement of Leatherface in front
of a line-up of "ghost" Glendales.
It's not a fake, just a creative player having some fun with
us:
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