Not all vehicles starts on a parking. The Passenger Bus, the Catering truck, the FollowMe car and the Deicing truck all come from away. The only vehicles that starts on a parking are the pushback, stairs and the baggage loaders and only when you select deboarding, because if you select boarding, the baggage cars will come (empty) from away.
They start at their starting locations on a parking, and they approach the airplane doors. If you just landed, if you select the GSX parking before going there, they already there, but if you start on a parking, they will pop-up there.
