This is one of those questions that can be answered by a close reading of the text. If you haven't read the story yet, I highly recommend it; it is super suspenseful and haunting. Near the very beginning of the story, it states that the Drover family left London because of all of the bombings that had occurred in the city; it wasn't safe to stay there anymore. During World War II, London was aerial bombed pretty badly. A lot of families left to go to the countryside to live until things settled down. However, Mrs. Drover needs to return to their home in the city; the text of the story says that she returns "to look for some things," but never gets more specific than that. She goes alone, and enters the house, which has been locked and sitting there for quite some time. Everything is dusty and the air is "dead," and she had a hard time even turning the key in the rusty lock.
It is at this point that the story really picks up, because sitting on a table in the hallway is a letter, addressed to her. She wonders who could have gotten in to put it there, and then the story takes off from there, leading us into her past, and ending in an ominous future.
I hope that those thoughts help; it's a great story well worth the read. Good luck!