https://itunes.apple.com/us/app/traffickcam/id1067713017?mt=8

Police find an ad for paid sex online. It's an illegally trafficked underage girl posing provocatively in a hotel room. But police don't know where this hotel room is -- what city, what neighborhood, what hotel or hotel room. This is where the TraffickCam phone app comes in. When you're staying at a hotel, you take pictures of your room... The app logs the GPS data (location of the hotel) and also analyzes what's in the picture -- the furniture, bed sheets, carpet and other visual features. This makes the hotel room identifiable. Now when police come across a sex trafficking picture online, there is a database of images that may reveal which hotel room the picture was taken in.

Downside is that app user can probably be tracked (I'm not tech savvy, so I really don't know)