When it comes to "residual hauntings" there are many ideas out there. Starting in 1890, four different concepts of this idea have been put forth in an attempt to explain ghosts.
Cody Polston is an author, historian and ghost investigator. He is the author of multiple books on history and paranormal topics. He was the host and producer of the popular podcast Ecto Radio and a writer for Ghosthunter X magazine. He is one of the founders of the Southwest Ghost Hunter's Association and has been investigating paranormal claims since 1985.
In the 1990's several other ghost hunting groups gave him the moniker the "Hitman" due to his involvement in "debunking" several well known haunted locations in Texas. Although the nickname was intended to be a negative insult, Cody adopted the nickname and began using in on his podcasts and other media venues.