The Elder's keycard is a quest item in Fallout: New Vegas.
Location
The Courier can obtain the Elder's keycard by pickpocketing it from either Elder McNamara or Hardin (depending on how they completed Still in the Dark), in the Brotherhood of Steel bunker located in Hidden Valley. It can also be obtained by killing them.
Quests
This is one of the 3 keycards required to start the self-destruct sequence during For the Republic, Part 2, Render Unto Caesar or The House Always Wins, V and Wild Card: Side Bets.
Notes
This item is treated as a quest item and thus cannot be dropped.