In some cases, there are items that are supposed to be hidden for the player to explore and actually search. What I think would be a good reward to teach players to search would be a flag for pickupable/interactable items: "Does not show up on press of ALT key".
Yep wonderful, exploration has to be worthwhile and doing what you said makes it so. If it's optional that's okay. I also like hidden areas and secrets which take a lot of skill to find and cannot be found without persistence. Exploration bonuses are sometimes good too and to keep it logical to story perhaps someone can pay you to find something hidden or map an area.
