If there are 20 people in a coffee shop, then there are at least 21 cameras: One embedded in each phone, and, usually at least, one tucked high in the corner. What you say may be overheard and tweeted; you might even appear in the background of another person’s selfie. But that doesn’t stop even the most privacy-wary people from entering coffee shops. They accept the risk inherent in entering a public place.

This notion—of a “reasonable” expectation of privacy—guides researchers hoping to observe subjects in public. But the very idea of what’s reasonable is a complicated one. Faculty at three universities are currently facing criticism after creating databases built using surveillance footage of students as they walked through cafes and on college campuses. You might reasonably expect being overheard in a coffee shop, but that’s different than suddenly becoming a research subject, and part of a data set that can live forever.