Sentence:  The <WSD>detective</WSD> kept them in view. He made his way casually along the inside of the shelter until he reached an open scuttle close to where the two men were standing talking. Eavesdropping was not a thing Larard would have practised from choice, but there were times when, in the public interest, he had to do it, and this was one of them. 
Meaning of the word marked with <WSD> tags:  A police officer who looks for evidence as part of solving a crime; an investigator.