question about a passage
【In Dallas both warrant officers were destined to play crucial roles— Bales, because a hospital emergency room is the worst possible place to establish secure communications; Gearhart, because the bagman was a stranger to agents of the Vice President's Secret Service detail. In Washington, however, they were merely two names to be checked off a long manifest.】
1.Bales and Gearhart are president's secret Service in charge of the "the black bag". why the agents of the Vice President's Secret Service doesn't konw them? and what does that "detail "mean?
2.what does "they were merely two names to be checked off a long manifest"mean?