There have been quite a lot of data breaches in the past and I wonder how the companies got to know them. Were they only finding it in the news? Did individuals inform them?
What about small data breaches, e.g. if you have a user who has an account that was hacked but the exact same (email, password) combination is used in my service. Is it possible to get to know that and thus block the user?
If you have thousands of image/document files, is it possible to have something like a google alert to get a notification if the document has been shared (e.g. by creating a hash of the file)? Are there services providing that already?