Curious, was "F/P" space available for sale? Is it possible the other passenger paid the difference in his/her Economy Class ticket to the First Class fare? I've been known to be on the upgrade list myself (at the very bottom) and look in my Sabre GDS to see First Class "F/P" inventory available for sale and pay the difference in fare. I don't consider it jumping the upgrade list if I pay for the difference in fare.