private detective
noun /ˌpraɪvət dɪˈtektɪv/
  /ˌpraɪvət dɪˈtektɪv/
 (also private investigator)
 (also informal private eye)
- a detective who is not in the police, but who can be employed to find out information, find a missing person, follow somebody, etc.
私人侦探 - She hired a private detective to follow her husband.
她雇了个私人侦探跟踪丈夫。  
 - She hired a private detective to follow her husband.