Box cutters on 5th 9/11 plane

I wonder why when talking about September 11, nobody ever talks about the flight from Toronto to New York that was supposed to take off that morning, but was grounded due to the immediate ban on air traffic that was put in place after the initial attacks? The airplane was owned by Air Canada. Since it didn't go to New York as scheduled, the airline scheduled it a few days later to go to a destination within Canada. Passengers putting their luggage into the overhead bins found box cutters taped to the insides -- just as with the four flights that were hijacked. So there was a fifth flight planned, which means 4 would-be terrorists remained in Canada. Wonder what happened to them?