Image: Bend Studio / PlayStationOfficial screenshot from Bend Studio / PlayStation.
Finders Keepers is a Bronze trophy for picking up your first collectible. Some collectibles unlock through the story itself, so almost every player gets it in the first hour.
Play the opening missions and it will pop. If not, read any sign or pick up any collectible marked in Survival Vision.
Collectibles carry over into free roam, so there is no rush on the later ones.