I realise that a penalty should be self assessed if the player thinks the ball moved. My question though, does not relate to this actual incident. The rule as you quoted it is correct. But because the ball was out of sight, 8" below top of grass, there was no way to know whether it first of all moved and if so, did it return to it's original position.

I was asking what a rules official SHOULD rule in a case where there is no way of the player or his competitors knowing if the rule was infringed or not.

This happens in many cases when a ball oscillates - we often place our club behind the ball and see a slight oscillation - Did the ball return to it's original position or not - I don't think anyone can say. In friendly games we let this go, but wonder what would happen if it was brought to the attention of a rules official.