The scheduled flight that was cancelled might be due to one of the airlines other planes at Seattle going unserviceable and the crew repositioning the aircraft to fill in for the unserviceable aircraft and not using ADSB during the repositioning flight.
It might also be due to N449QX having a fault that needed rectification at Seattle where there might be better repair facilities. This would also explain why it was at the freight & maintenance area rather than the passenger area.