Anons we need to dig on this. Looked it up myself. Sure enough N449QX departed SEA for YYJ the morning of 10 Aug. The return flight was cancelled.
Question: How did N449QX get back to SEA-TAC later that evening to be hijacked around 8PM PDT?
Recall witnesses said he taxied from the cargo ramp not the passenger gate ramp.
https://flightaware.com/live/flight/N449QX/history/20180810/1637Z/CYYJ/KSEA