The CID is a part of the British police: the detectives who look into very serious crimes — murder, big robbery, that kind of work — rather than the officers in uniform who walk the street. CID stands for Criminal Investigation Department. In news and crime stories, someone “in the CID” is a detective, often in ordinary clothes. They are police, not spies. Though actually, not every British force still paints those three letters on the door, but in learner English and in stories CID still means those detectives. Other countries have detective branches too, and some reuse the same letters, but this word is filed as the British one.