In the 4th season of Maria Watches Over Us, it's revealed that Toko was adopted. In the final episode, when she and Yumi went on the half-day date reward which Toko won for finding Yumi's Card, Toko reveals that her real parents died in a car crash which she was the sole survivor of, and she was later adopted by the daughter of the head doctor of the hospital, who happened to be a classmate and friend of Toko's biological mother.
Toko tells Yumi that she had suspected for a while that she was adopted from overhearing "cruel adults". However, it has only been confirmed recently in a couple of episodes earlier. During the scene where Suguru Kashiwagi is driving Toko, he gets angry that Toko has found out and starts questioning her who opened their mouth, naming 3 women (I can't remember their names, but I believe they were the ones who tried to shame Yumi during the 3rd OVA Season). However, Toko doesn't answer, being more scared of Suguru's aggressive speeding.
So I am wondering, is it known who told Toko she was adopted, or did she found out herself?