Just because the top of the battery is completely flat doesn't mean it is maintenance free. Look carefully at the edge of the top - there might be a notch where you can lift up a cover to expose the cells.
Formerly, only gel or AGM batteries were sealed and there was no need to ever check water (because there was no water to check
Perhaps the new Optima batteries are really sealed with no maintenance - I have no experience with them. I seriously doubt if yours are maintenance free.
Welcome to the forum - glad to have you aboard!