- You may be looking for the novel of the same name.
A corpse marker was a robot deactivation disc. They were often red and iridescent. They often had some sort of inscription or logo on them to identify the company that made them.
During one spate of murders in and around Kaldor City, they were forced into the hands of human victims. They were a clue the Fourth Doctor and Leela used to help solve the mystery of the murders. (PROSE: Corpse Marker)