The Helsinki, Finland airport will be the first in the world to track passengers to within feet. The plan is being hailed as a technological breakthrough, and is drawing scrutiny from privacy advocates. The sensors will monitor all mobile phones with Wi-Fi access turned on from parking lot to takeoff, helping to observe crowding and prevent bottlenecks at the two-terminal airport. Passengers opting in through an application will also receive offers from shops and restaurants, as well as gate and flight information. Such systems are being deployed more widely as physical shops seek to remove a disadvantage against online retailers who are more advanced in analyzing the digital traces left by customers. About 150 white boxes the size of a Web router are set up around the airport to collect the unique identifier number called the MAC address from devices searching for Wi-Fi networks. They then scramble the information and send it onward to servers. The complete system, in place by the end of the year, may allow a delicacy shop to alert those walking by about a reindeer jerky offer or tell a traveler to Tokyo that there’s just enough time for sushi before she needs to start walking to the gate.