Citigroup is testing Bluetooth beacon technology in a pilot that gives customers cardless access to branch ATMs after business hours, among other services.

The beacons, small devices that communicate with mobile apps over Bluetooth, effectively turn mobile phones into "keys" that unlock the door when they sense a customer is near, removing the need to fumble for bank cards to gain entry to the ATM vestibule.

Mike Anzola, Citi's vice president of mobile product management, likened the feature to keyless car entry, where doors unlock by sensing the driver's proximity.

The beacon "doesn't know anything about the owner of said key, it just knows that it's in sight," he said. "The same applies for us. The phone and the app are the key and [the beacon] is like the ignition and battery that help operate the whole thing."

Beacons are more commonly used in retail settings to inform shoppers of discounts as they navigate a store. A large department store may use beacons to provide an offer that is specific to the department the shopper has just entered, for example.

Citi has been testing beacon technology with iPhone and Apple Watch owners in New York City since mid-March. The final rollout to the 10-branch pilot was the week of May 9, according to Deirdre Leahy, a spokeswoman for the bank.

Customers that opt in to the pilot can also receive "contextually relevant" location-based notifications if they're within 30 feet of the branch, Anzola said, adding that the range actually varies depending on environmental factors Citi is taking into account during this testing phase.

For example, Leahy said, in the winter, some branches offer customers hot apple cider when they come inside. With this technology, passing customers would receive an alert informing them of the option to warm up inside with a hot drink.

"We want to make it fun, we want to make it useful for people," Anzola said. "It's not just a question of serving marketing messages to people — I think that's the fear," that customers could feel spammed with notifications, especially in densely populated New York, where one could conceivably pass three branches in just a couple of blocks.

"There's a frequency limit on all of these [notifications], so if you work across the street, you're not going to get this message every hour or every day," Leahy said. And the customer can choose not to acknowledge the notification, Anzola added. "It disappears on its own, you won't have this huge thread of notifications."

How long the pilot runs will depend on customers' responses. Leahy said 450,000 customers are currently eligible to participate, and those who have opted in so far have done so without aggressive prompting by the bank.

The beacons are made by Gimbal, one of the largest beacon providers. Gimbal's other clients include South by Southwest, the NFL and the U.S. Open.

Inside Citi "smart branches," beacons are also placed underneath the workbench desks, where employees and customers transact together on computers or mobile devices. When the beacons sense a customer has been waiting too long for a teller, it can send a notification directing the customer to a workbench.

"We can tell how long you've been waiting. We don't know who you are or anything about you, we just know there's someone there that's lingering," Anzola said, adding that the team strives to keep notifications "very contextual in nature" and find the "delicate balance" of maintaining customers' comfort levels.