So many scenarios fall into a grey area with regard to where to mark the location. Had you intentionally displaced the spider, I would say it is clear you should mark the location where it was found before entering the car. That would be how to treat an observation of, say, an injured wild fox that is caught and taken to a wildlife rescue centre, or a butterfly that is caught for the purpose of pinning in a museum collection; even if the photos are taken in the rescue centre/museum, you would mark the location as the capture site, ideally.
However, in your case, the spider has travelled, albeit unwittingly, of its own accord. And, in any case, any location that you were to mark other than the one where you first sighted the spider would be a guess, since you cannot say for sure where it entered the vehicle. I would put an explanatory note in the observation comments though, with reference to the location where you suspect it was transported from.