Seeing five open seats means nothing. People often have confirmed seats without a seat assignment. And a flight at F0 doesn't mean there aren't any F seats available, just that they're not selling any more F. There may have been a cancellation and they have to deal with moving people from a different flight, or the Coach cabin could be oversold and they may have decided to leave those seats available for upgrades only to avoid IDBs, or a number of other reasons. If you've already checked in, you could have been upgraded on your PNR, which doesn't show on the upgrade list (that's the bug where you need to be offloaded and check back in).