I've had all kinds of shenanigans happen with upgrades....mostly good but once bad.

I too have seen empty seats in FC when I was told no upgrades on a SEA-MSP flight. I was sitting in Row 14 and even asked the GA when he came on the plane and was told by him again there were no upgrades. The FA told me they were not allowed to move anyone, so I used GOGO with it's free access to Delta.com to send a complaint email in flight. Upon my arrival in MSP I had an apology email explaining I should have been upgraded and after a few exchanges I ended up with a $100 eCert and 5000 miles. I think in this case it was a bad GA.

I've also been already sitting in my exit row when someone came in with the same seat with the FA behind him carrying my new boarding pass.

I've also been upgraded as they scan my boarding pass.

My last flight from DTW-SEA there were 4 empty seats in F, and I can't believe there weren't 4 other medallions on board.