Brotherhood Missions are essential part of the game purpose of which is to reward players for team work.
A player can earn up to four Reward Packs during a single Mission, namely: Basic, Standard, Elite and Exclusive Packs. Reward Packs are stored in the Smuggler's Den > Special tab and can be opened during 72 hours from the moment when a player received them.
After 72 hours the Reward Pack expires and a player will not be able to open it anymore. Make sure to open yours every day to maximize the rewards.
Unlike personal Quests and Tournaments, a player receives the Reward Packs not when the Mission ends, but at the moment when Brotherhood fulfills requirements for each Pack. It means that you can open the Reward Packs when the Mission is still in progress. Often players are getting confused because when the Mission ends a list of unlocked Reward Packs is displayed again, even when a player has already opened all Packs.
If you can't find the Reward Packs in the Smuggler's Den > Special tab, it means that you have already opened them earlier.