Originally Posted by flyerCO
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.
In my case, I was told that all but 2 F pax had checked in (and F would be full when they did), and that there were more than 2 elites ahead of me on the wait list. My suspicion is that I was somehow already in F, and by some odd coincidence no one noticed.