In every single level of Bad Piggies, there is a place where the camera stops looking at Ross and stays in position. This is the edge of the level. Sometimes, there will be an infinitely high rock/ice wall in which nothing can get past. (See glitches and bugs for things that can). Other times, (like in Road Hogs level 1 and 3), there is nothing there to stop a contraption from exiting the screen. This is because it is right at the start of a level or after the finish line. The contraption can keep on going for eternity, or the level will automatically restart.
The contraption can also fly out of the view of the camera just by going up for a long period of time. This is not possible for levels such as Field of Dreams as they have a rock 'ceiling'. Sandboxes are generally larger and wider so the boundaries are further apart.