Does every zone have secret encounters after you finished them?
Not so much secret encounters, just stuff you can't do on the first visit.
Town has alot. Sex with Ellena, experiments with Elly, boob and ass training with Vivian (unless ass is your starting fetish, ass training is very hard to trigger), Amber's bondage scenes, Demon Layer whip, milking with Belle, Mara/Galvi/Beris content in the Bar
Forest has Dryad,
1st dungeon has Vivi
Mountain has Jora and Manticore
2nd dungeon has Feng, Katsune, Bed-chan threesome
Caverns has Tabitha's house
Labyrinth doesn't really have new interactions after clearing it. Just a quest to do nightmare mode anywhere. (Pet Ushirs' Tail)
Capitol is unfinished So.....