Object Permanence

The way that most  infants minds work is, out of sight is also out of mind. This is until they reach a milestone, Object Permanence that they realize the object still exists even if they can't see it. There are a few steps in reaching and understanding object permanence. The first is:
Birth to 1 Month:

• the infant can't follow objects. The eyes will develop over time.

1 Month to 4 Months:

• They eyes can follow the moving object but they can't see it if it is not there.

4 Months to a Year:

• Infants will search for a missing object in the place where they object usually appears.

1 Year:

• Child learns the concept of object permanence and search for a hidden object.

Of course the times on these are not see in stone each kid is different. But this is way when you play peek-a-boo will some children they start looking for you being your hands or some babies love they game because they can't see you and it's exciting for them when you jump out from around your hands.

Until the Infant learns this concept, nothing exists out of the field of vision.