Jail keys are used in the Heist minigame to free members of the robbing team from jail, or to let out members of the guard team early without making them wait for Internal Affairs to release them. These may be pickpocketed by robbers from original guards, and once it is stolen, the robber can no longer pickpocket, and the guard can no longer release their teammates early.
Robbers that get turned into guards will not give a key should they be pickpocketed.
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patch 13 October 2014 (Update):
- The examine text on the Heist jail key has been corrected.