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Living without store loyalty cards still won’t keep you under the radar. Smartphones, which many people consider necessities, broadcast exact locations about every six seconds to the wireless carrier.

Stores also can detect the location if the phone is searching for a wireless Internet connection, allowing retailers to market to potential customers in the neighborhood.

“If it’s set to look for Wi-Fi, your phone sends out a signal every few seconds saying, ‘Hey, I’m phone 1234.’ It sends out a unique phone identifier,” Mr. Brookman said. “Stores can detect you’re nearby and maybe send you a coupon to go in.”

These types of deals may be convenient for some, Ms. Urban said, but even they don’t come free.

“It looks like a free benefit, but it’s really in return for our data,” she said. “To make that choice well, it’s important for the consumer to know what it means to give up that data in return for that benefit.”

Although there are ways to browse more securely and try to maintain some anonymity, Ms. Urban said, constant data collection is becoming unavoidable online and off.

“It’s actually just a feature of life right now in digital life and non-digital real-world life that information is being collected,” she said. “And more is being collected all the time.”