In the Panessa Studios there are, in total, fifteen surprise boxes left by Harley Quinn. Destroying all of them will lead to solving of three new Riddler's riddles (the register of the solved riddles rises after each fifth destroyed box).
To destroy a box that you just found (they can be on the floor or on desks) use a batarang. The box will explode at the moment when the batarang touches it.
Better hidden boxes to be destroyed
Box number 4 from the map that is added to this chapter is located behind the weakened wall in the main area of the Backstage. Get to that item from the northern side and destroy the wall by using the explosive gel.
Boxes number 11, 12 and 13 are in the side rooms of the stage B. In order to get there you need to use the openings in the walls or the short ventilation shafts.
Box number 15 is in the small room next to the stage C of the movie studio. Start south from the room with the secret and find a ventilation shaft, because you will have to go through it.