侦探(zhēn tàn) - detective, investigator in Mandarin

侦探 (zhēn tàn) is a detective. It covers the private investigator of fiction and the plain clothes officer who works cases, and it is the word in 侦探小说 (detective novel) and 私人侦探 (private detective).

It works as a verb too, meaning to investigate covertly, though that use is mostly written. The everyday verb for investigating something is 调查, which any police force or newspaper would use.

The trap is assuming a 侦探 is a policeman. Chinese police ranks do not map onto the British detective constable and detective inspector, so a serving officer is 警察 or 刑警. Reserve 侦探 for the private investigator and for Sherlock Holmes.