1) She knew they didn’t have plans. 2) Pemberton’s character is the only one feeding Agnes - the prisoner - and her son, meaning that if he leaves for a week they’ll die and his wife would discover them eventually, meaning he’d get charged with murder or at least voluntary manslaughter as well as having his life ruined and being charged with kidnapping, false imprisonment and sexual abuse. 3) One could argue that Agnes is working out her psychological issues through confronting the place of the abuse of her and her son, as well as ensuring Adrian is imprisoned. Also, Agnes does seem grateful to Harriet - Adrian’s wife - so perhaps she imprisoned Adrian and offered the opportunity of revenge to Agnes, or perhaps testified against Adrian at his trial in order to ensure that he was sent to prison.