The Red Door stuff got me thinking. There's a scene in the movie "Taken" with Liam Neeson where the young girls were taken through a red door on Rue de Paris or something like that. Couldn't find much on this other than the comments at the end of this article. Kinda thin but, sure makes me wonder if red doors are a marker.

https://parismoviewalks.co.uk/update-taken/