Currently, bonus boards are “discovered” as you get close and then appear on the map but are not “collected” until you smash them. Barn finds and treasure chests have the added “search area” until you actually discover it but then the same thing applies as the bonus boards.
If you own the Treasure Map DLC, when you activate it, it should only mark items that are currently “discoverable” at that moment in time as “discovered”, and adding the location on the map.
This means that if a barn find is not “discoverable” yet (IE, it’s not yet “rumoured”), then it’s status is not changed. Once it becomes “discoverable”, you need to activate the “Treasure Map” again, which will mark anything new as “discovered”.
You could also go one step further and have a menu that appears asking you WHAT you want to “discover”. So you can discover all boards, but not barn finds.
Credit also goes to Burny0077 for final assistance on wording 