Following his work on Bigfoot, Paulides' next project was Missing 411, a series of self-published books and two documentary films, documenting unsolved cases of people who have gone missing in national parks and elsewhere.

According to Paulides, his work on this subject began when he was doing research in a national park and an off-duty park ranger found him and expressed concern about the questionable nature of some of the missing person cases which occurred in the parks. The ranger knew Paulides' background and asked him to research the issue. Paulides obliged, and asserts that he uncovered multiple lines of evidence suggesting negligence on the part of the park service in failing to locate the missing people. He broadened his investigation to include missing people from across the world, and this led to his belief that he has uncovered a mysterious series of worldwide disappearances, which he said defied logical and conventional explanations.

As of August 2021, Paulides has written at least ten books on this topic. According to A Sobering Coincidence, he does not yet have a theory on what is causing the disappearances, although he indicates that the "field of suspects is narrowing." Paulides advised his readers to go outside of their normal comfort zone to determine who (or what) is the culprit.

Paulides' books publicized the fact that the US National Park Service does not keep an independent list of people that go missing in their parks. While there is a database for incident and criminal reports, it is not widely or consistently used and it doesn't interface with other criminal databases. In response, a petition was created to make the department accountable.

The interest in the book series prompted the creation of a documentary film based on the Missing 411 books; this film was released in 2017. Images of maps made by Paulides regarding his theory have been frequently shared on social media. The theory has also gone viral on TikTok.