This is amazing. About 40% of my dream content consists of exploring little interstitial secret spaces like this, and if you're REALLY nice, I might tell you what some of my private mental ones look like. They absolutely fascinate me in real life, too.
http://www.thisiscolossal.com/2017/08/secret-moveable-work-studio/
@zebratron2084 oh wow. oh wow. yessss. wow. (my "secret passage behind the wall" is like this; it's stable enough that I've found it in multiple dreams, leading to and from different places, usually involving a warehouse space that is being interstitially and illicitly inhabited, mostly by artists)