When a corpse is looted, its weapon is currently available as a separate item for pickup as well as being linked to the corpse when the main inventory is opened. If the player would like to take an attachment from the dropped weapon, there isn't an easy method to do so, and requires the players weapon to be dropped before taking the weapon from the corpse to remove the sight.
It would be very useful if the linked weapon could be opened as a sub container in the same manner as the vest or rucksack. Any attachments that the weapon has could then be moved to the players inventory without having to juggle weapons.