What is the importance of "keys" is the novel, The Witch Elm?

Throughout the novel, French uses the languages of locks and keys to symbolize the secrets which are yet to be revealed and to symbolize the boundaries of Ivy House as a refuge. The most relevant key is the key to the garden which unlocks the door out into the alleyway. There were two keys to the garden, one of which had been missing for several years before the events of the novel. The missing key was what Dominic used to gain access to the Ivy House garden from the alleyway, symbolizing the secret penetration of the Ivy House refuge for Susanna. During the police investigation, the focus on this missing key highlights the illusory nature of Ivy House as a refuge.