In 2006, French artists, Julien Berthier and Simon Boudvin created a small building facade with a door to nowhere on an empty wall in Paris. They called this project Les Specialistes.
It was designed using Parisian architectural codes to look completely timeless and authentic and took 30 minutes to glue to the wall.
It remains there today seamlessly merged into the neighborhood.
No one has questioned its appearance or ever seen the door open, yet it remains there today, 4 years later...and it is regularly cleaned by city services...
I found this documented on-line via: