It was in the pre-release, but not in version 1.0?
(http://klikfusion.com/snapshotpro/FiNCK_WhereIsDoor.png)
It was right between the two shown here.
It has been moved to a more dificult to reach place. i think.
It involves some boxes and the special area.
Actually, Firecat, you're incorrect.
The middle door has moved to the tutorial section. There was only one door there, now you go in, and there's another one there too.