This could apply to simple text analysis too. some agent looking for now disappeared person X collects a bunch of their known comment, email, chat text, search

This could apply to simple text analysis too. some agent looking for now disappeared person X collects a bunch of their known comment, email, chat text, search pattern text and looks for certain distinct words, word patterns and so forth among these things then searches for similar correlations across a broad, maybe global swath of general chat, social media, email, comment, search posting and also correlates some of the sites involved with keyword similarities to easily identify X as Y even though Y has completely disconnected from anything having to do with having been X