I’d hate to see any sort of writing you’ve done. You have to build suspense before you announce what’s on the other side of a mystery door.
It’s easy to use a crowbar in a rental place without damaging its fixtures.
Obligatory horror movie clip
Hey, is there any sign of another door on that side of the laundry room storage area?
Nope. That’s why I started measuring things.
For the love of God, Montressor!
We have a shaft going from under a kitchen cabinet to a small door in the laundry room downstairs. Since it is above the laundry room, I wonder if it is a laundry drop that was closed up.
There’s at least one other person here who’s familiar with Coraline, seeing as post #15 references button eyes.
open it open it open it open it open it open it open it
What I meant was, whoever did the remodeling may have wanted to move the wall, but couldn’t if it was weight bearing, and that’s how he got stuck with a 3’x6’ space.
Is this an apartment in a building that used to be a large house, and was subdivided? Is there any evidence of shelves or heavy furniture having once been there? It could be the secret passage to the kitchen. Or, if there is an unused landline plug, then it may be the secret passage to the conservatory.
Ah, but I said I would say I just pried open a secret door, not “Just pried open a secret door and found a dead rat and half a piano”. 
I would give out vague clues and stretch out it out for at least five or six pages…
I’m going with stairs down to a basement/sub-basement (depending on how “bottom” you are) that was deemed unusable. Or a secret smuggling tunnel.
Any chance the house was in the Underground Railroad-- wait, you’re not in the US. Are you in Amsterdam?
Stairway to the basement?
Do you hear unexplained noises? Soft, wet bumps in the night? Perhaps something that doesn’t quite sound like a human sigh, but more like a chuckle that turns to a sob?? Perhaps an impatient rustling sound, of something trying to walk while dragging one foot?
Just asking.
Have the old people next door given you stinky jewelry, or offered you chocolate mousse that tasted funny?
It’s the Secret Treasure Room!
Its probably not …the entrance to John Malkovich’s head.
Also, if you do open it, put your piano in it and close it… what if it comes alive?
Call Geraldo Riverra
Can you ask the upstairs neighbors what occupies that space in their apartment?
My guess, given that you are in the basement, is that the area that is now your apartment used to be something else, and the walled-up area was the entryway to a separate something.
If the building is a converted large house, the original configuration might have been a kitchen and a housekeeper’s rooms. For a small-ish, older apartment building, a pre-dryers laundry area with drying racks and pulleys, and a manager’s apartment. Something larger – maybe commercial space and a backroom area.
Is there more space in the basement than you have described?
clears throat
Post 15.
I’ve decided that I am going to open the door. I will post the results tomorrow, possibly…
I’d ask the folks upstairs what’s above your empty space. It could be the space where plumbing pipes pass through to/from upstairs. What kind of heating do you have? It couldn’t be where the furnace or water heater is because sooner or later, you’ll need access to them.