But most of the two billion Mastercard holders aren't aware of this behind-the-scenes tracking. That's because the companies never told the public about the arrangement. 

Alphabet Inc.'s Google and Mastercard Inc. brokered a business partnership during about four years of negotiations, according to four people with knowledge of the deal, three of whom worked on it directly. The alliance gave Google an unprecedented asset for measuring retail spending, part of the search giant's strategy to fortify its primary business against onslaughts from Amazon.com Inc. and others.

But the deal, which has not been previously reported, could raise broader privacy concerns about how much consumer data technology companies like Google quietly absorb.

"People don't expect what they buy physically in a store to be linked to what they are buying online," said Christine Bannan, counsel with the advocacy group Electronic Privacy Information Center (EPIC). "There's just far too much burden that companies place on consumers and not enough responsibility being taken by companies to inform users what they're doing and what rights they have."

Google paid Mastercard millions of dollars for the data, according to two people who worked on the deal, and the companies discussed sharing a portion of the ad revenue, according to one of the people. The people asked not to be identified discussing private matters. A spokeswoman for Google said there is no revenue sharing agreement with its partners.