It seems to have been imagined in order to associate it with the mysterious Whispers which some superstitious people claim to have heard.

No old castle is complete nowadays without its ghost, so we have for our share the Lady of Glencardine and the Whispers," he laughed.
"But I thought it was a matter of authenticated history that the Cardinal was actually enticed here, and disappeared!" exclaimed the girl.

"I should have thought that the Glencardine papers would have referred to it," she added, recollecting what Walter had told her.
"Well, they don't; so why worry your head, dear, over a mere fable?
I have already gone very carefully into all the facts that are proved, and have come to the conclusion that the story of the torture of his Eminence is a fairy-tale, and that the supernatural Whispers have only been heard in imagination." She was silent.

She recollected that sound of murmuring voices.

It was certainly not imagination.
"But you'll let me have the key of the cabinet, won't you, dad ?" she asked, glancing across to where stood a beautiful old Florentine cabinet of ebony inlaid with ivory, and reaching almost to the ceiling.
"Certainly, Gabrielle dear," was the reply of the expressionless man.
"It is upstairs in my room.