Pizza tracking: A spike in coupon redemption at PizzaLand at lunchtime is revealed in San Francisco by FortiusOne and Appcelerator’s Titanium+Geo package.

The approach could also allow for much more targeted advertising. “When you understand when and where [a person is when they use an app], then you understand context,” says Scott Schwarzhoff, vice president of marketing at Appcelerator. “If it’s 7 a.m. and a person is in the marina district, then you understand where that person lives.”

Targeted ads would allow developers to command higher rates for advertising. But this kind of data could also reveal things that users might rather keep secret.

“It’s like Minority Report,” says Byung-Gon Chun, a researcher at Intel Labs Berkeley. “As Tom Cruise walks by the billboards, they change their advertisements based on his presence.”

Chun, who studies mobile security, recently coauthored a paper showing that many Android apps share user data, including location information, without making it clear to users.

Chun and colleagues Jaeyeon Jung at Intel and William Enck at Penn State University developed a program called TaintDroid that examines the data that Android apps pass to the Web. Half of the apps they analyzed transmitted geolocation data, most without asking for permission or making it explicit in their documentation that they would.

“Even if this disclosure is in the [end user license agreement], it’s hard for users to figure out what kind of information the app is actually sending,” says Chun.