Happens in YYC too. Not uncommon for AC850 to LHR or AC844 to FRA to have CBSA Officers at the gate questioning passengers after the Gate Agent boards them. Can happen as often as once a week. Usually has to do with currency... passengers have to declare if they are exporting a certain amount of cash and not all of them do. Sometimes the flight is delayed if CBSA denies travel to a passenger and their bag has to be pulled.