Currently in the Sims 4 there are two hidden maps, they both come with the default game.
The Hidden Grotto in Oasis Springs and Sylvan Glade in Willow Creek.
Very little information is written about them within the Prima e-guide, they are simply given a mention. Everything we know about them has come from player experiences.
In order to access the Forgotten Grotto you need to visit any lot within the neighbourhood plot 2 in Oasis Springs. Turn the camera angle around and you’ll see a little cave. This can only be opened by someone who has maxed out (level 10) on the handiness skill. Once it has been opened, any sim can visit at any time regardless of their handiness skill level. You can bring any sim to the lot while visiting, but this does not unlock the map for them if they haven’t already been there.
If your sim has level 10 handiness skill you will need to explore within the cave. The game will pop up some options for you to complete. Selecting the correct options will grant you access to the map.
Wide Path, Climb Ladder, Step onto Ledge, Travel to Grotto.
Within the Forgotten Grotto you’ll be able to search for frogs by clicking on the water pump, there is a toilet near the entrance however no other amenities, some plants, a few places to fish as well as rocks and crystals to dig.
Anyone can gain access to the Glade, visit any lot within neighbourhood plot 6. In the north east side of the neighbourhood along the canal, you’ll see a large, decorated tree that allows you to click on it. In order to unlock the map, you need to select the correct pathway. Each sim will need to do this in order to visit the map. You can bring any sim to the lot while visiting, but this does not unlock the map for them if they haven’t already been there.
Follow the Sound, Travel Downstream, Enter the Mist, Travel to Glade.
The map has a very mystical feel to it. While on the map you’re able to search for frogs, fish in the water and in a pond, you can also harvest some of the plant life located on the map.
The purpose of the maps appears to be to provide a popular place for rare spawners of fish and rocks within the caves.