OOH ad firm Titan has installed 500 Bluetooth beacons in payphone booths across New York City.

The beacons, first spotted by BuzzFeed, are manufactured by Gimbal – a proximity marketing firm which was founded as part of Qualcomm Labs but went solo this May. They have actually been in place for as long as a year and are reportedly concentrated in the central and lower areas of Manhattan.

While the installations were approved by the city's Department of Information Technology and Telecommunications, BuzzFeed has raised concerns about the lack of both a public approval process and transparency from the city's agency.

There's no word if these beacons have so far been used to run ad campaigns.