Object permanence can be taught.
If the bottle is put within the infant’s reach so he can get it, all is fine. But if the bottle is put in the infant’s reach in a manner such that it is masked (like under a pillow) one of two things may happen.
The infant will either simply just cry; or the infant will pursue the bottle under the pillow. In the former case, the infant is demonstrating a lack of the concept of object permanence. For him the bottle has gone. In the latter case, the infant retains the image of the bottle and knows it still exists under the pillow even if he can’t see it; and wanting it, he goes after it.
This demonstrates an awareness of object permanence.
You can make the task less difficult for the child who can’t retain the image by partially hiding the bottle.