Evan Schuman's StorefrontBacktalk

“We’re using the Android device as a sort of radar that detects the customers as they walk in. The wireless module of that Android device is used to create an active scanning area within” the store, said the vendor’s CEO, Ido Gaver. “When the customer launches the app for the first time, aside from creating the user account, it’s associating their mobile device parameters with their profile.”

LoyalBlocks estimates that it can deliver accuracy of about five inches, a spokesperson for the vendor, Loren Pomerantz, said.

But Mood also spoke of its ability to send tones out from those speakers and have an app listening for these high-frequency bursts of sound. Given that each sound will announce which speaker it’s coming from—some stores have hundreds of these speakers in each location—and that the app can detect the relative strength of those sounds and, therefore, extrapolate the relative position to each speaker, it can theoretically locate shoppers with an unusually high degree of precision.

Even better, given that these speakers already exist within quite a few of the largest chains—Mood customers include Walmart, Target, Home Depot, Sears, Toys”R”Us, Foot Locker, Abercrombie & Fitch, TJX, CVS and Macy’s—this has some practical, relatively low-cost potential.

Downsides: The phone not only has to be activated with the app launched, but it can’t be in a pocket or anywhere else where the light would be blocked. That means the shopper can’t accidentally hold it with a finger covering the phone’s camera lens. For phones that have two lens—front and back—the finger-blocking is less of an issue, because the app should work as long as both lens are not blocked.

On the plus side: The phone can be in airplane mode—no network—and this light-based app should still work.

For what it’s worth, Ryan said he’s not concerned about his offering’s limitations. He envisions it being used to track shoppers while they are actively interacting with the mobile app. If that’s the case, then there’s no reason why the phone would be off or in someone’s pocket or for a finger to be covering the lens (or both lens).

That’s a valid approach, and it would certainly apply to the Special K scenario described at the beginning of this story. But it underscores the differences in the approaches and how retailers need to think through exactly how they want interactions to happen. The fully effortless scene of a customer walking into a store and being identified when his or her phone is in a pocket is just one possibility.

And to be fair, there are a huge number of caveats with all these approaches. First, the legal issues surrounding such shopper tracking are still unclear. One federal appellate panel has ruled, but we have yet to hear from the full circuit. Nor has the U.S. Supreme Court, which already has a case relating to this topic before it, weighed in. There are tons of state legislatures, too, not to mention thoughts from Congress and various federal agencies.

Then there is the question of how much tolerance—or resistance—for such efforts shoppers will offer. Given how little people seem to care about privacy—through their purchase actions, not what they tell people taking surveys—it’s most likely that retailers will have free reign to take location tracking as far as each chain wants.

Critically, this assumes such tracking is used to deliver true benefits to shoppers. Amazon has always been the gold example of this tactic. The E-tailer customizes its pages and sends tons of E-mail pitches but is so focused on truly appearing helpful that it generates almost no resistance. If this helps customers get in and out of stores more quickly and helps to rapidly locate hard-to-find products, this could be a huge win. Assuming, of course, at least one of these technologies proves to actually work in the field.