Fan Zhang, the owner of Happy Child, a trendy Asian restaurant in downtown Toronto, knows that 170 of his customers went clubbing in November. He knows that 250 went to the gym that month, and that 216 came in from Yorkville, an upscale neighborhood.

And he gleans this information without his customers' knowledge, or ever asking them a single question.

Mr. Zhang is a client of Turnstyle Solutions Inc., a year-old local company that has placed sensors in about 200 businesses within a 0.7 mile radius in downtown Toronto to track shoppers as they move in the city.

The sensors, each about the size of a deck of cards, follow signals emitted from Wi-Fi-enabled smartphones. That allows them to create portraits of roughly 2 million people's habits as they have gone about their daily lives, traveling from yoga studios to restaurants, to coffee shops, sports stadiums, hotels, and nightclubs.

"Instead of offering a general promotion that may or may not hit a nerve, we can promote specifically to the customer's taste," says Mr. Zhang. He recently emblazoned workout tank-tops with his restaurant's logo, based on the data about his customers' gym visits.