It Could Have Been Her (original) Lisa Jewell

Jane Trevally is walking her dogs on her country estate when a small white terrier appears, alone and with no sign of the teenaged girl he’d been staying with nearby. When the teenager is reported missing, Jane offers to return the dog to his registered owner, hours away in London. Arriving at a run-down house called Thornwood in the deepest backwaters of Hampstead, she is immediately on alert—because Jane has a dark history with this house.

The man who answers the door is not the man that Jane remembers from her past. He is cagey, and claims to know nothing about the missing teenage girl. Then, through the window of the house, Jane catches a glimpse of a haunted-looking woman.