Collected the following secret scrolls to unlock the corresponding bonuses:

SCROLL - BONUS
Scroll 1 - Hidden World: Professor's Villa. Found in level 2 in the professor's mechanical box.
Scroll 2 - Hidden World: Abandoned Tenement House. Found in level 3 near the crowbar in the old district.
Scroll 3 - Hidden World: Collector's Room. Found in level 4 in upper right-hand wall of ruins.
Scroll 4 - Hidden World: Ruined House. Found in level 5 in an alcove to the right of the tomb entrance.
Scroll 5 - Hidden World: Shop. Found in level 6 at the base of the fountain.
Scroll 6 - Hidden World: Mysterious Temple. Found in level 7 to the left of the archway.
Scroll 7 - Hidden World: Cemetery. Found in level 8 in the rafters inside the villa.
Scroll 8 - Hidden World: Museum. Found in level 9 on the garden wall.
Scroll 9 - Hidden World: Old District. Found in level 10 next to the computer desk.
Scroll 10 - Hidden World: Gallery Basement. Found in level 11 in the gallery workspace on the shelves under the window.
Scroll 11 - Hidden World: Catacombs. Found in level 13 in a top-right grave in the first catacombs room.
Scroll 12 - Hidden World: Nun's Room. Found in level 15 on a bookshelf in the cardinal's office.
Scroll 13 - Hidden World: Hidden Clue. Found in level 16 at the top center of building.