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A corpse marker was a robot deactivation disc. They were red and iridescent. Aboard Storm Mine 4, Taren Capel had the sandminer robots he controlled place corpse markers on their human victims as a ironic gesture born from Capel's belief in robot superiority. (TV: The Robots of Death)
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)