The Enigma of Room 309

Title: “The Room That Wasn’t There”
A young man moves into an old apartment building after getting a surprisingly cheap rent.
Everything seems normal at first, except for one strange rule mentioned by the elderly caretaker:
“Never open Room 309 after midnight.”
The problem is that the building only has rooms up to 308.
The protagonist initially assumes it is an old man's mistake. But over the next few nights, strange things begin happening.
At exactly 12:09 AM, he hears someone knocking from the wall beside his bedroom.
Three knocks.
Every night.
One night, he follows the sound and discovers a narrow door hidden behind an old bookshelf. There is no number on it.
He opens it.
Inside is Room 309.
The room looks exactly like his own apartment—but everything is slightly different.
There are photographs of him sleeping.
His clothes are hanging inside the wardrobe.
His phone is sitting on the table.
And on the bed is a diary containing entries written in his handwriting.
The final entry is dated tomorrow.
It says:
“Tonight, he will open the door. When he does, I will finally get out.”