On the streets of tech-savvy South Korea, the latest and most cutting-edge gadgets flash in people’s hands the instant they become available.

Less visible are miniature spy cams that in recent years have led to a reckoning over Internet-based sex crimes that have devastated women’s lives, spurred scandals and ignited massive offline protests. The cameras lurk in clocks, clothes hooks, coffee cups and calculators, capturing women on the subway and in the bedroom.