Sartre said the instant you catch someone watching you, you become an object

Sartre described peering through a keyhole, absorbed in watching, until footsteps make you realize someone is watching you watch. In that instant, he argued, you stop being a free subject acting in the world and become an object categorized by another consciousness, judged, defined, caught, which is why other people's gaze can feel like a kind of threat to your freedom itself.

— Jean-Paul Sartre, Being and Nothingness (L'Être et le Néant) — 1943