At the end of last season, Edith decided she wanted her daughter back in her life, so she went back to Switzerland and reclaimed her. Before she went though, she had Mr Drewe meet her at Downton to say she had a "friend's child" who needed a home and asked him if he might take her in because she insisted the child could not be at Downton. Mr Drewe figured out then and there Edith was the mother, and offered to write a note to himself from a "dying friend" who asked him to adopt his child, so Mrs Drewe would not suspect anything.
Personally, I now think things might have gone smoother if Edith had let Mrs Drewe in on the secret from the start. But I guess she felt the fewer who knew the truth the safer she and Marigold would both be.