I think there are 2 options (pick your favorite)

1. The show creators hoped no one will look for the moment where Sheldon hides the ball.
2. There is a gap that is not shown in this scene. When Sheldon says he'll always have the dent to remember the ball by, we can see that the storage unit door is closed: However, we never see them open the door when they exit:
   Perhaps Sheldon hid the ball when he/ Amy opened the door.

I think there are 2 options (pick your favorite)

1. The show creators hoped no one will look for the moment where Sheldon hides the ball.

2. There is a gap that is not shown in this scene. When Sheldon says he'll always have the dent to remember the ball by, we can see that the storage unit door is closed:

However, we never see them open the door when they exit:

Perhaps Sheldon hid the ball when he/Amy opened the door.