For investigators tracking down sex traffickers and their victims, online advertisements shot in hotel rooms can contain critical clues – but because of the surfeit of ads these days, investigators need the public's help. Enter TraffickCam, a phone app that uses crowdsourced snapshots of hotel rooms to help law enforcement locate victims and prosecute sex traffickers. The European Parliament estimates that more than 20 million people worldwide are victims of trafficking, the vast majority for sexual exploitation.