Changing planes in Canada is not the same as entering it. If you stay inside the sterile transit area, or you are moving through a US preclearance area at the airport, you are asking to pass through rather than to come in. Step out of that sterile area and the picture flips: you must present yourself for examination right away.
(1)Subject to subsection (2), the following persons are not seeking to enter Canada but are making an application under subsection 15(1) of the Act to transit through Canada: (a)in airports where there are United States’ in-transit preclearance facilities, in-transit preclearance passengers; and (b)in any airport, passengers who are arriving from any country and who are transiting to a country other than Canada and remain in a sterile transit area.
(2)Any person seeking to leave a sterile transit area must appear immediately for examination.