This also comes down to confidentiality. When you hire a private investigator, they are acting under your instruction and if you don’t want the person to know they are under investigation the private investigator you’ve hired will try and keep it as low-key as possible. Depending on what you hire the investigator for will determine if the person being investigated needs be notified or not. If you are getting a private investigator background check on an employee, then the employee will need to be notified.