Many missions require a player to recover a specific item for the mission to be completed. If a player is unable to locate a mission item, then it is possible that the container which has the item is not displayed on the players overview. Right clicking on the active overview Tab and selecting the 'Load Defaults to Tab' -> 'All' will add all items to the overview, such as mission structures which may have previously been filtered. Sorting the overview by distance players can determine if any of the nearby entries may contain the mission item. The mission description should contain some hints as to the location of the item which may prove useful for players.
It is the responsibility of the mission owner to protect any mission critical items. If another player recovers the mission item before the owner of the mission, then the mission may not be able to be completed. Players can see if the mission items are available for purchase in the contract system, however any such purchase may not fill the requirements by the agent and is done so at the players own risk.