Furnesse disappeared in 1983, while walking his dog. According to the October 31st 1985 issue of The New Frontiersman, his disappearance was obviously connected to the disappearings of other notable people.
The theory was right, as he was brought to an island along with other vanished people, to work on a secret project which they thought was for a movie. He told Max Shea (a missing writer) that he worked on a real human brain (probably of Robert Deschaines) to achieve a special effect. He died along with everyone else on a Pyramid Deliveries ship that exploded.