Stores try to follow your digital trail, but you can hide your movements. By Miriam Cross, Associate Editor From Kiplinger's Personal Finance, September 2014 Shoppers know there’s a trade-off involved in signing up for a retailer’s loyalty program or app: discounts and rewards in exchange for information about you and your buying habits. But simply browsing in a store with your cell phone in your pocket can be almost as revealing. That’s because when a phone scans for nearby wireless networks or Bluetooth signals, it emits a unique code, called a MAC (media access control) address. Companies can read the string of characters and zero in on your movements.See Also: How to Keep Your Data Private This mobile eavesdropping is more widespread than you might think, says Pam Dixon, executive director of the World Privacy Forum. Most retailers are mum about tracking, but the practice is more common among those with multiple locations, she says. Retailers say the information helps improve store layout, schedule staff efficiently and shorten lines. Analytics firms that shadow these signals say your ID is protected because data is anonymous, aggregated with data from other shoppers, or both. But privacy advocates say that even without knowing a cell phone owner’s name, analysts can infer lifestyle interests based on how long shoppers linger in an aisle or how often they visit a location, and can trace these patterns for years. Advertisement You can limit tracking by registering your MAC address with the Future of Privacy Forum’s new opt-out platform at www.smartstoreprivacy.org. Unfortunately, doing so will remove your identifier only from the handful of companies that have agreed to comply. A better fix: disabling Wi-Fi and Bluetooth while shopping. And iPhone users may be pleased with a privacy-friendly feature of iOS 8, expected this fall, that broadcasts randomized MAC addresses to cover the user’s trail.