Does water leak from a small hole in the vinyl at the botom of a pool?

Are you lossing a lot of water? Do you use any type pool cleaner that attached into the skimmer that bounces on the pool floor? If you do and the base is poolcrete is being better down.. and THAT could cause a hole...

You may or may not have a leak but the sunken area is caused by the collapse of the sand or ground under that area of the pool allowing the liner to fill in the hole created by that collapse.