The French study has just been published last week [0]. It is based on a vast cohort study in which serum samples were collected regularly. They tested older samples and found some anti-SARS-CoV-2 antibodies. The article also mentions an Italian study with similar results [1].
[0] https://link.springer.com/article/10.1007/s10654-020-00716-2
[1] https://journals.sagepub.com/doi/10.1177/0300891620974755