Jennifer Wilson was dead, to begin with. There is no doubt whatever about that. More than a year ago, Sherlock himself had investigated her murder with, for the first time, John by his side. John was a doctor, after all, and John himself had examined her body regarding the cause of death. Her killer had been shot in the course of investigation, the files had been closed months ago.
Sherlock knew that Jennifer Wilson was dead. Of course he did. How could it be otherwise?
Therefore, when Jennifer Wilson all of a sudden appeared in his very own living room (well, his and John's to be exact), Sherlock found himself momentarily baffled.
His mind told him that this woman was (yes, do repeat it!) DEAD. Like in NOT alive. Like in not supposed to be here at all!
Nevertheless, she was standing right in front of him.
Sherlock knew he was not drunk.
He was neither high on nor coming down from any kind of stimulant.
Apart from the self-proclaimed-high-functioning-sociopath-thing he was suffering from no major mental disorder.
And he did not seem to have a particularly heightened body temperature - no fever hallucinations, then.
He slapped himself. Hard. And it hurt! He focused on the hurt for a moment and was sure he was awake as well.
She was not a doppelganger. Not only did he have an excellent memory for faces, she was also wearing the same clothes they had found her in when she had been murdered, the loud pink coat with matching shoes and nail polish, albeit the polish of the right hand was still ruined from her efforts of scratching that message of hers into the wooden floor of the abandoned house, while literally struggling for her last breath. No way she was a doppelganger.
Using his own method of elimination of the impossible he stared at the remaining possibilities. Jennifer Wilson was dead. She was also standing just a few metres away on the carpet.
Therefore she had to be a -
"Hello, Mr. Holmes!"
She - or it? the ghost? the she-ghost? - had an interesting voice. Sherlock had naturally never had the opportunity to talk to her or even hear her voice before. After all, she had been DOA - Dead On Acquaintance. He had to smirk at that little morbid private joke, courtesy of John after the doctor had come home from the pub once with just a quarter of a pint too much. Anyway, here she was, and come to think of it... this was kind of exciting! Sherlock Holmes had never had the opportunity before to talk to any murder victim AFTER having solved the questions concerning their murder. The questions relating to her sort-of-presence here and now were certainly worth some consideration on their own merit as well.
So. He threw her another assessing glance and decided that he might as well go along with this experience - or whatever it was.
"Miss Wilson." He made sure his face was deadpan. "So glad you managed to stop by! If you require a seat, please do help yourself!"
Nice handiwork of yours, planting your cell phone onto the serial killer so we could trace him via GPS. Scratching the password to your account into the floor and all that. Really neat! You seem to be less of an idiot than the majority of the human species, congratulations.