NW's system is set to upgrade elites into F once inside the 30min checkin window if paid/already upgraded F pax have not checked in. Depending how many people don't checkin it's possible to be third in line and have the system do the upgrade. I was once the lucky last plat to get upgraded on a flight because of this. The agent might not know this has happened until they scan your BP and it beeps to let them know you've been upgraded.