Is it ethical to use data you overheard in a conversation?
Without understanding your research topic and methods, it is hard to say objectively if you can use the data. That said, the American Psychological Association (APA) classifies naturalistic observation as exempt from the informed consent process. This does not mean it does not require approval by an ethics board (i.e., an IRB). Naturalistic observation studies are valid in a wide range of fields including Medicine, Psychology, Zoology and Ethnography. That said, as with any research methods, it takes skills to conduct and interpret naturalistic observation studies properly and within the framework of their limitations. This seems like a nice starting point: http://peace.saumag.edu/faculty/kardas/Courses/RMPA/naturalisticobservation.html