As an Express guy, the only way I have been able to "list" before showing up at the gate is when I have gotten rolled from one flight to the next. Got to the gate for the next flight and new agent tells me I am already listed for the jumpseat. Looked at the list myself and sure enough, there I was. Apparently it depends on how the agent does it but sometimes the jumpseaters roll over with the other SAs. Could he/she have been rolled from the last flight of the night to next flight in the morning?

Only other thing I can think of is that jumpseater has a friend who is an agent and had them do a "favor".