Your long ago asked question was a good one. Darned if I knew the answer. I asked the rules expert at our state association (Golf Association of Michigan) and she was not sure. Finally I asked the USGA. The USGA response was that it is presumed that the ball moved downward
even if it did not appear to move from its position. Since the ball moved downward, the player caused the ball to move. Therefore, the player incurs a one stroke penalty and the ball must be replaced - see Rule 18-2a.
Personally I don't like the idea of presuming the ball moved when close observation did not indicate any movement. However, the USGA is always right so now you have the answer. You correctly took a penalty for presumably moving the ball.
Lots of luck replacing the ball to its original position. Which of course if you did not do so, the result is playing from a wrong spot and another penalty