What were O. Henry's inspirations for this story?

Hence the houses of this district, having had a thousand dwellers, should have a thousand tales to tell, mostly dull ones, no doubt; but it would be strange if there could not be found a ghost or two in the wake of all these vagrant guests.

This quote from "The Furnished Room" suggests that O. Henry was in the habit of inventing stories to suit settings. A neighborhood and a partiicular rooming house such as he describes in detail in "The Furnished Room" could suggest characters and situations to a man with his remarkable imagination. In this case it is a young woman who has apparently vanished and a young man who is trying to find her. The spooky landlady is the third character necessary to complete the tale.
It is challenging to look at some of O. Henry's other stories and guess whether they too emanated from a setting and the unique mood which that setting evoked. He may have seen a man smoking a cigar in the darkened doorway of a closed hardware store and invented the characters in "After Twenty Years" to populate the stage that was already set for them. Why would a well-dressed man be standing in a darkened doorway in a deserted neighborhood? He must be waiting for someone--but why there?
O. Henry's story "The Last Leaf" begins with a description of a different New York neighborhood in the same perceptive and evocative manner as that of the red brick district of the lower West Side in "The Furnished Room."

IN A LITTLE district west of Washington Square the streets have run crazy and broken themselves into small strips called “places.”

A bum sleeping under a mound of newspapers on a bench in Washington Square may have inspired the creation of "Soapy" and his efforts to get himself thrown in jail.
Another great short story writer who derived as least some of his ideas from settings and the moods they evoked was Guy de Maupassant (1850-1893). In his excellent story "Minuet," the French author creates two old people who are living relics of the opulent era before the French Revolution. The man was a dancing master and his wife was a leading court dancer. They come every day to the Jardin du Luxembourg in Paris because it reminds them of their youth and the ancien regime. At the request of the man who is narrating the story, they dance the minuet to show him what it was like. The dancing master tells the narrator:

"The minuet, monsieur, is the queen of dances, and the dance of queens, do you understand? Since there is no longer any royalty, there is no longer any minuet."

Maupassant created the whole story out of nothing but the bare setting of the Jardin du Luxembourg, where he liked to walk in solitude and think about story ideas. Like O. Henry, Maupassant was a prolific writer who was always under pressure to meet deadlines. There is no surprise ending to Maupassant's story. He did not always surprise his readers, as he does in his famous story "The Necklace." At the end of "The Minuet" the elderly couple stops dancing, and the illusion of being back in the time before the French Revolution abruptly vanishes.

They suddenly stopped. They had finished all the figures of the dance. For some seconds they stood opposite each other, smiling in an astonishing manner. Then they fell on each other's necks sobbing.