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".
How about the best of both worlds?
Using the alt key to highlight obvious items like we know from DD, etc. like loot and/or visible items on a table.
Adventure games use a key to highlight objects in case you stuck. Such a 'hint' key for harder to spot objects could be simply another key.
Then you can decide yourself if you want just convenient help with the ALT key or when you are frustrated or just too lazy to explore you can use the 'hint' key.
It would be just nice if they would be not right next to eachother
