private detective, PI, private eye, private investigator, operative, shamus, sherlock

someone who can be employed as a detective to collect information

• hotel detective, house detective, house dick — a private detective employed by a hotel or retail store
• inquiry agent — (British) a private detective
• store detective — a private detective employed by a merchant to stop pilferage

• detective — an investigator engaged or employed in obtaining information not easily available to the public