Besides criminal investigation, there are other kinds of investigation: journalism investigation, insurance investigation, social worker investigation, etc. Due to the nature of investigation, sometimes the subjects shouldn't know that they are investigated, i.e. cannot give informed consent to allow investigation on them. Furthermore, the investigator may also need to lie or pretend to be a neutral person or even a trust one. They may even have to put secret recorder. I wonder what is the limit of them? How are they regulated? Should the government make specific laws for each occupation, and thus if you don't have a license to do such job, you are not allowed to investigate?
I live in Vietnam, if that matters.