No you fool! It will let the ancient old ones enter our dimension!
Nope, my story still works.
Inept Painter: Huh, door stuck. 
pulls knob nothing happens. :dubious:
grabs knob, braces foot against doorframe, pulls. Nothing happens. :mad:
grabs knob, braces both feet against either side of door, pulls with all his might. Doorknob comes off in his hand, IP propels himself across the room and embeds head in opposite wall. :smack:
Next painter just fills in the doorknob’s hole with putty and paints over it. 
Note: Add to possible contents: 1 doorknob.
Note #2: Search wall opposite door for head-shaped patch, approx. waist height.
Except that the OP said all four walls are within his flat/apartment. So it can’t be space used by an adjoining apartment. And surely not all four walls are load-bearing. (Plus even if a wall is load-bearing, you can still cut a door in it.)
There’s probably a boring explanation for why the room is walled off, but still worth pursuing.
I’d open it. Or at least drill a hole big enough to get a look-see through. Whatta ya think? Open it?
We gots to know…
No handle or knob on this side? Might mean that it’s only an exit, which means that, whatever comes through it, they want it to stay on yout side of the door.
It probably just a small, oddly shaped closet. My house is old and has several of them. One of them was even painted shut when I bought the place. It proved to be an empty, dirty little cubby that abuts one of the (now disused) chimneys. In my upstairs hallway, there is a sealed door that, if you could open it, would take you into a bedroom closet. All of this kind of stuff is a result of a former owner doing some remodeling, himself, back in the 1970’s. Although the quality of his work wasn’t bad, he did make some very odd choices.
There’s a door like that in the bedroom. If it opened, it would go directly into the living area. That one isn’t as mysterious as the unused space.
Don’t drill a hole and look through, that is just a horror story waiting to happen. Ugh, gives me the chills I don’t like eye poking stuff, just don’t
I have no idea what may be in there but the optimists among us will assure you that when that door closed, somewhere another opened. That means somewhere in this world there is someone looking at a door, the universal opposite of yours, thinking “Why in the heck will this door not close?”.
You must have seen the opening few minutes of “Horrors of The Black Museum.” See it with a friend.
If I were in this situation, my subject line would’ve read “Just pried open a secret door…” 
Seriously, get a putty knife and a rubber mallet and start cutting through the paint, tapping the knife with the mallet if needed. After that, get a pry bar and see if you can pry it open.
Like Dung Beetle said, Open it open it open it AND THEN COME BACK AND TELL US!
This made me giggle.
Open it!
No, no, don’t listen to her: the Cheshire Cat will simply vanish into the alternate universe, leaving us to suffer the wrath of He Who Shall Remain Un-named.
Seriously, a pry-bar? In a rental dwelling?
I opened the painted-shut old windows in my rental dwelling without damaging them. I used a hammer to pry them open, but a small crowbar would work. If you’re careful, there shouldn’t be any damage that couldn’t be repainted. 
How about if he gets a small camera probe and drills a hole big enough to fit that and a light through so he can get a look at it without any potential eye-poking terrors? If it turns out there is something evil in there he can quickly seal over a small hole and then quickly move out.
It’s the world where everybody has buttons in place of eyes. (Seriously, am I the only one here familiar with Coraline?)
I am assuming that 3 x 6 is the surface area of whatever is behind the door, in which case, what are the dimensions of the door?
My guess is a connecting passageway that was walled off in a remodel. But without a sketch of the floorplan, it would be hard to tell, really. I like the stairway concept.
Upright piano storage! Bonus: Free piano!
Just a standard door, as if it opened into another room.
It would passage straight into the storage area for the laundry room.