Police around the country are using a powerful but relatively inexpensive cellphone tracking tool to solve crimes. And in some cases, they have used it to track people without a search warrant. All mobile devices are assigned what's called an advertising identification number, a unique code that allows apps with location services to target consumers with promotions. For as little as $7,500 a year, Virginia-based Fog Data Science offers a service called Fog Reveal that uses that ad-ID to track a device's wanderings, when location services are enabled.