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It's one thing for e-commerce stores to track your behavior as you shop online, but now real-world stores are following your every move through your cellphone. That's right: It's not quite that scene at the Gap in Minority Report, but the next time you leave your iPhone's WiFi signal on in public, expect to have a digital trail.

As The New York Times's Quentin Hardy reports, over 100 brick-and-mortar stores — including big ones like Home Depot and Nordstrom — have paired up with the shopping behavior watchers at Euclid Analytics, who can learn a lot just by following your wireless connection:

Using the information, retailers can tell whether someone walked by the store, whether a customer came in and how long the visit lasted. If it is a big store, with a couple of Wi-Fi antennas, the owner can start to see where in the store someone went.

If you think about it, this in-store transition really isn't that different from Amazon dropping some cookies on a user's browser history and tracking their only shopping and browsing habits. Following shoppers down the aisle, by way of phone, doesn't differ too much than seeing where someone clicks around on, say, the Urban Outfitters website. It may ultimately be less invasive, but you're not signing up for it either.

Expectations and discomfort can trigger privacy violations, even if it's in the fine print of an Apple end user agreement. Being watched by random marketers as you walk into their stores, whether it makes your shopping experience better or not, is not something cellphone users expect. Similarly, when Facebook bought Datalogix in order to better understand its users offline buying habits, it surprised many users because it was taking their expected privacy concerns where they least expected to have to worry about them. The same goes for this new offline tracking innovation in retail: Stores aren't supposed to be doing that.

There's also the very legitimate concern that Euclid Analytics has a lot of information on its hands. "Euclid has more data than it gives to customers," explains Hardy, alluding to the trove of data inside each phone — not so much your files as the GPS tracker everyone with a smartphone has unwittingly put in his pocket. "It gives its customers only anonymous data in a collected form, so individuals won't be targeted." Euclid also requires stores offer an opt-out feature for customers, but when was the last time you noticed that while cruising racks at Nordstrom?

In the future, these might change from opt-out to opt-in, which sounds a lot less threatening. People who want the perks of an "efficient" location-based marketing service can let stores watch them beeline to the tampon aisle. Until then, however, if this kind of thing weirds you out, it might be a good idea to turn Wi-Fi off each time you head to the mall. Or else it's hello Mr. Yamamoto, indeed:

This article is from the archive of our partner The Wire.

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