Researcher anonymity in fieldwork
This problem appears to have been considered by the social science community, and at least some resources have been prepared.  In particular a little searching turned up a book, "Fieldwork, Participation and Practice: Ethics and Dilemmas in Qualitative Research" by Marlene de Laine, which appears to discuss these sorts of issues---the Google Books preview only showed me so much, but there was at least some discussion of covert fieldwork.