If you read the article it's clear they were tracking people for weeks after they left the airport: "The document shows the federal intelligence agency was the

If you read the article it's clear they were tracking people for weeks after they left the airport: "The document shows the federal intelligence agency was then able to track the travellers for a week or more as they — and their wireless devices — showed up in other Wi-Fi "hot spots" in cities across Canada and even at U.S. airports. That included people visiting other airports, hotels, coffee shops and restaurants, libraries, ground transportation hubs, and any number of places among the literally thousands with public wireless internet access. The document shows CSEC had so much data it could even track the travellers back in time through the days leading up to their arrival at the airport, these experts say."