I feel like this has been known for a long time. For example: If you walk into a Kindergarten class and watch the children play, once they notice you watching them they change behavior away from "natural play" to "observed play". I believe Cory Doctorow made this observation a spell ago.
Edit: I'd like to add that one of my parents was a teacher in a school with two-way mirrors for observation. People could secretly observe a given class in session either for observing the teacher and//or observing the students live but without the "observer effect". The entire school building was designed for this purpose and whilst everyone knew it it appeared to work as intended. "Out of sight is out of mind" is real. Yes, this particular parent was on both sides of the glass.