Okay, first off: Next to nothing in Witcher 3 is really hidden. Most of it is in plain sight, marked on your map. Even worse, none of the stuff you find is really that interesting. I like the game, but this is a complete hit and miss in this context. (Some of the TES games, especially the older ones may be a good fit though)
Some recent games I want mention:
- The Bard's Tale IV: Barrows Deep has lots of hidden stuff. And not placed at random. Some of that stuff is hidden behind puzzles, some is hidden behind secret walls / etc. Mostly accessible via using the right skill to reveal it (you always get the required skills though). What I like here is the non-random placement and content of the secrets. Also, no wall-hugging required. On the down side, you may end up using those skills compulsively.
- Some other more recent dungeon crawlers that may be of interest: Legend of Grimrock, Vaporum. Also hand-placed, so that's nice. But some of these secrets are… oh well… rather obscure. No good chance to find them without a guide. This is a very common issue in this genre. An issue that the new Bard's Tale avoids, mostly.
- Pathfinder Kingmaker: Lots of hidden stuff, although mostly hidden only via skill checks and being close enough to something. I'm not sure this is what you are looking for. If you are, Age of Decadence may also be worth a look.
- Elex is definitely worth a mention. Exploration works well, and there's lots of actually interesting stuff to find.