There are two hidden lots. One inside Willow Creek and one in Oasis Springs. Haha. Just now realized I called Willow Creek - Willow Springs in my post. Oopsie!
To find Sylvan Glades in Willow Creek you will need to have your Sim travel to Crick Cabana. It's a lot with a home on it. Once you are there go to the right of the home and look for a huge tree
Once you find the tree, click on it and select "View". After viewing click on it again. This time select "Water", this may need to be done 2 times. Then click on the tree again and select "Discuss Nature" again you may need to do this a couple times. Now click on the tree again and select option to "Explore". Once your sim enters into the tree you will get pop up messages. These are the answers you want to choose:
1) Follow Sound
2) Follow Down Stream
3) Enter the Mist
Your Sim should now appear in Sylvan Glades.
The other one is Oasis Springs is called Forgotten Grotto. To find this hidden lot travel to either Dessert Bloom Park or Affluista Mansion. You will need to have handiness level 10 in order to enter into this hidden lot. Once at Dessert Bloom Park, have your sim go the the right and almost all the way to the back of the park. When he/she arrives at the entrance, click on the boarded up door and "Break Open" once the barrier is cleared click on it again and "Explore".
These are the answers you will want to choose:
1) Take Wide Path
2) Climb the Ladder
3) Step onto the Ledge
To find out what kinds of rare goodies your sims can score at each of these hidden area's, go to Carl's The Sims 4 Guide
(plus while your at Carl's why not check out the cheats that are available to use. A sly simmer, TwistedMexi, discovered ton's of "secret" cheats that the developers used and were shipped with the game!)