The figures come from a client of mine who had conversations the last few days with senior US government Customs/Immigration officials.

People are crossing day and night across the entire border. In a lot of places there are no Canadian Police Officers or Customs Officers present. But when they cross in one of these uninhabited places a sensor is tripped.

US Customs employs numerous automated sensors across the US/Canada border that register people crossing the border. Their sensor network is more extensive than Canada's. There are automated video cameras that are tied to these sensors. The network was originally designed to track drug traffickers crossing both ways.

It is these sensors that track people moving over the Canadian border. They are there to track people trying to move from Canada into the US but are also used for the reverse.