This is not an ethical practice for any researcher working with human subjects.
When sociologists do research on human subjects, they must obtain the informed consent of those subjects. In order to do this, they certainly must not misrepresent themselves. They cannot even simply fail to identify themselves as sociological researchers. They must identify themselves and inform their subjects as to the purposes of the research that is being done.
A sociologist who does not identify him or herself to research subjects cannot possibly obtain their informed consent. Failure to obtain informed consent is a major breach of research ethics.