The Scavenger+ skill gives Aloy the chance to loot extra resources from downed machines by introducing a new potential loot item, the Machine Scavenger Box. With the Scavenger+ skill learned, machine kills have a 20% chance of dropping this loot box which contains an assortment of machine resources.
The Machine Scavenger Box will only contain items that could otherwise be looted from that particular machine. For instance, a Machine Scavenger Box that is looted from a Watcher would never contain an Echo Shell because Watchers do not drop this resources.
If the player wants to gain additional resources while foraging natural resources, it not affected by this skill, but by Gatherer.