I'll start of by saying you made the story suspenseful. VERY, suspenseful. At some points I was at the edge of my seat, leaning in close to the computer. I was getting so worried for Joan. When she jumped, I jumped. And the whole idea of a figure standing in the doorway like that...creppy.
Read the story now
Now, getting down to technical aspects....this is where I feel you fell short. The sentences didn't flow all that well. At some points they were very choppy. And in one paragraph, you started every sentence with a word ending in "ing." It sort of distracted me from the story.
Also, the ending didn't feel very satisfying. Bushing it all off as dream just sort of seemed...anti-climatic. It would have been okay to end it like that, had you, let's say, made the mysterious mover come back one more time. Or maybe revealed to us that the set of keys was missing. You just needed something to make the reader at the end go "Oh, that definitely couldn't have been a dream."
Overall, the plot was really good. The story fell short in some areas, but I feel you have real potential as an author. Keep writing!