Let's say you are outside a room and the door is open. You cast Flaming Sphere inside, on a square you can see through the door and then move away so you no longer have vision of the sphere.
On the following turn, you choose to sustain the spell. When you do, can you roll the ball, for example, 10 feet to the right, even though you have no vision of the ball nor the place you are moving it to? What if you can still see the ball but you can't see the place you are moving it to, but you know the dimensions of the room? And what if you don't know the size of the room, can you move it 30 feet to the right? What happens if it hits a wall?