In my instance above in December, when I cleared the standby list I was listed as "FIRST, 15B, [can't remember boarding zone though]"

SDS doesn�t seem like the answer here because there is no SDS for Paid FC fares (unless FC is completely full) as same fare class availability isn�t required for FC so SDC will always be available as long as there�s open seat inventory available in FC. It�s possible it could be someone without a seat assignment in First although that is rare as seat assignments are almost always available to select ahead of time (with the exception of select D1 flights where some are blocked off until the gate) but it can happen. The best way to tell if there is pax without unassigned seats is to do a dummy booking and see how many FC seats are left to sell. If FC is only showing 1 seat left available for purchase but the UG list in the app shows 2 seats unassigned then someone has ticketed but not selected their seats yet (again rare but it can happen).

Did you really think the gate agent owed you an explanation relating to the fares, travel arrangements etc. of another passenger?