Each stage has three mementos with a total of 33 for all campaign missions. They’re mostly hidden in hidden areas that can’t be discovered easily without the help of a guide. Once picked up, they’ll be permanently registered to your collection so even if you repeat the stage, you won’t have to go all your way to go to the memento’s spot to pick it up again. This is also applicable for mementos that you have picked up right before you died or reloaded a checkpoint. For the individual location of the mementos per mission, refer to the list below.
Near the end of the first trench by the beach, continue to the end of the trench then enter the small makeshift MG nest to find the knife stuck by the opening.
While clearing the concrete bunkers, you’ll run across this closed door where the memento is located. Clear the marked bunker ahead then backtrack to this location to find it open. You can still backtrack to this location even after clearing the bunker with the mortar team in it.
Near the end of the mission where you have to clear the farm house and destroy the cannon, you’ll find this memento inside the barn.