Strange Disappearance

A young woman has disappeared from a fifth avenue home. The bachelor owner, Mr Holman Blake, says he knows nothing of her. But the housekeeper Mrs Daniel is worried and goes to the police. Q finds blood on the windowsill and the girl has not taken the time to pack. It seems clear that she left unwillingly. But why go over the roof when she could have walked out the servant door? (Goodreads)