Last year, Google announced the Store Sales Measurement service, through which Google said it collected approximately 70 per cent of US credit and debit card transactions through third-party partnerships, without saying who those partnerships were with.
The way Google's ad-tracking tool works is like this: If you are online and click on an ad for, say, a backpack, but don't purchase it, but then later walk into a shop and buy that same backpack, Google will know.
Both Google and Bloomberg say the data is anonymised, meaning purchase histories can not be linked back and used to personally identifiable information, including your billing address, name, age, or other details held by the two companies.
If you provide a Google email address when you purchase something in-store, the merchant will let Google know if it has a partnership with the firm.
In May past year, Google quietly launched a tool called "store sales measurement" that could help it determine whether an online ad led directly to real-world purchases.
If that doesn't assuage the uneasiness you feel over your real-world purchases being monitored, you can opt out of ad tracking by going to Web and App Activity.
One of the most recent examples of Google going to arguably questionable lengths to acquire data is its deal with MasterCard.
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For Google, this is just another step in bridging the gap between online ads and offline sales. Though Mastercard has been confirmed to be a part of that 70 percent metric, there are likely other financial service companies in the U.S. that have participated as well.
But most of the two billion Mastercard holders aren't aware of this behind-the-scenes tracking.
When confronted about the Bloomberg report, Google officials denied any partnership with MasterCard. It makes sense that Google wants this information-it gives them the ability to prove to customers that Google's ad service drives sales.
A Google spokesperson didn't comment on the deal with Mastercard specifically but said that before launching the beta product previous year, they developed a double-blind encryption technology to prevent both Google and its partners from viewing their respective users' personally identifiable information.
Mastercard denied suggestions that its data could be used to identify exact purchases.
Mastercard told the BBC it offers its own "media measurement services" to retailers, in which the merchant provides advertising campaign details and it supplies spending data for the duration of the campaign.
"This raises serious concerns regarding the use of private financial data", said legal director Myles Jackman.