Well basically "Hidden" refers to the visibility of an object in a room to the player's perspective
Some libraries have different meanings for 'Hidden': some removes the object completely, some move the object to another room, so you need to provide the library name that you imported
The default "Hidden", which is the "Visibility" is sufficient if you wanted to introduce a secret object, as long as these conditions are fulfilled:
1. The name of the object is unique to all other objects in the same room
2. The object is a stand-alone (not inside or on top of other object)
To count for the found secret object, use Attributes and add it by 1 for each object found
By the way, please ask the question clearly so people will understand