I was on a flight from ORD to DSM on Sunday. I am a 1K that was booked in 1st on a later flight and I was on standby for the earlier flight. 1st class on the earlier flight checked in full according to the gate agent but I was able to get on the plane in E+. Just before the doors closed the gate agent came down and noticed that 2 seats in 1st class were empty. I was expecting her to ask me if I wanted to move up. No luck. The doors closed and we flew to DSM with 2 open 1st class seats.