I have a half-finished hidden door bookcase sitting in my garage. I never could get the molding to look right, so it was always too obvious that it was a door. I always meant to go back and figure out a way to make it work, but I never did. I think I just need to change to a different type of hinge.
I just looked up the thread I made on the project and my reaction was "Holy crap, that was 2007" I didn't think it had been that long.
Anyways, here's the old thread for prosperity, maybe one day I'll actually finish it. http://forum.arcadecontrols.com/index.php?topic=64170.0
My father was a carpenter so I remember more than a few houses with some hidden surprises. Some of the ones that I can recall.
A crawl/storage space between the (steep sloped) roof and the wall. The access panel actually had two light fixtures mounted. To open it, you simply pulled on the light fixtures. I discovered it purely by accident when the owner was moronic enough to leave the panel aside. The panel was held in place by magnets and the magnets completed the circuit. Probably wouldn't pass inspection now that I think about it and probably unwise to leave the switch powered on when you're working around it.
A hidden hatch panel that opened up into crawl space between the walls. Climb the ladder and it led you into a hidden area in the attic. Basically, you had to, IIRC, start on the 1st floor, crawl past the 2nd (and 3rd?) floor until you reached the attic. I don't remember how many floors it went up. When I was older, I used to have nightmares about this one transposing that house onto my fathers workshop so the details are sketchy at best. Not sure why I had nightmares about it.
A faux rock wall that opened up into a simple shelving unit.
A disused dumbwaiter whose doors were paneled over in dark wood giving the illusion nothing was there. Kind of dumb IMHO, but whatever. IIRC, the house later burned to the ground.
The house I grew up had this feature (trim was added last year so the illusion is ruined now), but I recently saw a similar door in an upscale neighborhood that put what my father built to shame. The siding (on my house was set at a 45 degree angle, on the upscale house vertical) on the house went through the door. In other words, the siding was installed
on the door itself. With no trim and only the door handle to indicate any sort of opening. However, on the upscale house, the damn door was twice as wide and, by my estimates about two floors high. Except for the lone door handle and the stair steps, I had no idea the door was even there. I also strongly suspect the entire side of the house opened up to allow direct access to the
front yard pool. I've tried to find the house again but to no avail. The neighborhood is confusing as hell.

A house that actually had two subbasements. Access to the first was gained by a simple stair. Access to the second was done by pushing the stairs up to reveal another set of stairs going down. How one manages to get furniture down something like that is beyond me.

It was unfinished at the time, so I don't know if it was any concern.
Not really a "secret" but I encountered a mirrored wall that opened up in an accordion fashion. Not really a secret because you can see the spaces between the mirrors and wall. It just didn't have any trim. I always thought that was an interesting idea to pursue.
That kind of stuff always fascinated me. Probably why I love places like Winchester Mansion so much.