The idea that even if something is out of sight, it still exists is called

The idea that even if something is out of sight, it still exists is called object permanence.
This is something that is developed in humans over time - young children still don't have the notion of object permanence. This is why peekaboo games work well with children - because when you hide your face, the child doesn't know it is still there behind your hands because there is no concept of object permanence yet.