On a flight once - CX from HKG to JFK via YVR. The entire plane had to deplane in YVR while the cabin was cleaned - a bit over an hour. We sat in a secured holding area for that gate - people could enter but not leave. There was an Immigration Canada fellow there with a younger fellow. The younger fellow was preboarded (with the Immigration chap escorting him to the boarding counter). They processed his credit card and on the plane he went. The immigration agent waited at the jet bridge until long after I boarded.

The young fellow had no belt and no shoe laces. My best guess was he was on a one-way ticket out of Canada. Nobody on the plane with him though - and when we got to the US he cleared customs before I did with no issue. Not sure if he was deported, but the no shoelaces/belt was a bit of a clue.