https://www.theguardian.com/technology/2018/apr/06/facebook-using-secret-tool-to-delete-messages-from-executives

Facebook says change was made after mass data breach but lack of disclosure has angered users

Facebook has been using a secret tool to delete messages sent by its executives from the inboxes of their recipients, without disclosing the deletions to the recipients – or even recording that there was ever a message there in the first place.