Membership Leak Tracker is a WordPress plugin. It helps trace your site’s files (PNG, JPG, GIF, PDF, RAR, ZIP, XLSX, DOCX etc) shared on the internet via P2P softwares and other piracy sites.
It performs its task by Embedding user (who downloaded from your site) info into files before zipping them. It is not like watermark. It hides info in way that it is not visible to humans.
When you find some file shared on the internet, you can upload that file to Membership Leak Tracker plugin and it will show info of the user who downloaded this file from your site.
It has encryption feature to encrypt sensitive user info so only you can decrypt and view it. Please view screnshots for more details.