In the Rabo arc of the anime only, Hiyori mysteriously regains her memories, even though they were shattered. In what at first seems like a Deus ex Machina, she breaks Nora's memory spell by herself after she happens to catch Yato's scent. But, according to psychology, smell is the most important human sense when it comes to creating and recalling memories. Because Yato's scent is incredibly strong to ayakashi or (in Hiyori's case) half-ayakashi, it must evoke a strong reaction of memory within Hiyori.
Hiyori regaining her memories of Yato and Yukine by smelling Yato in the anime holds some credibility since familiar scents are often used to help people who are recovering from amnesia.
