Leak Detector tells you whose copy a pirated file, or some text copied from one, came from.
Every copy made in File Watermarker, every ARC sent when you approve readers in ARC Tracker, and every file a reader downloads from a Reader Magnet has the reader's name stamped on the page. It also carries hidden marks in the file and in the book's own text that point back to that one copy, and they stay even if someone crops the stamp off.
To use it, open Leak Detector and drop the pirated PDF or EPUB on the page, or paste at least 200 words of text copied from it, as it is and without retyping. It checks straight away, in your browser, so the file is never uploaded. A match shows who the copy was made for, the book and the date, with an Add to Piracy Monitor button that logs it or adds it to a listing you already have.
What it can't find: copies made before the hidden marks were added carry only the visible stamp. EPUBs carry the most, and the marks survive copy and paste. PDFs carry less and can lose them if printed to a new PDF or converted. Retyped or scanned text carries nothing.
A match is evidence, not proof. It shows whose copy it was, not who shared it, because a copy can be passed on. Ask before you accuse.