My brother lived in an apartment in Chicago for a couple years. Apartments there are usually the whole floor on the thin but deep building. It took him almost a year before he noticed there was a full size door behind his refrigerator. He never bothered to figure out where it went as that was the edge of the building(external wall). So it was just weird. That apartment was full of stuff like that. It had another door that looked like it was from Willy Wonka(about 1/8 the size of normal). There was also an attic hatch. The typical drywall on a square frame section of trim. The issue was it was only like ~18” diagonal. He figured it was like an access panel to plumbing or something…but nope. Just went to the attic but you could really only stick your head through unless you were a child.

He had all sorts of interesting places. One had the old school elevators where you had to pull the grates down to close the shaft and elevator opening. You could just look up and see multiple floors above you. Had the slide lever for up/down instead of buttons.