Rule 18 - Ball at Rest Moved
Rule 18-1 By Outside Agency
If a ball at rest is moved by an outside agency, there is no penalty and the ball must be replaced.

Note: It is a question of fact whether a ball has been moved by an outside agency. In order to apply this Rule, it must be known or virtually certain that an outside agency has moved the ball. In the absence of such knowledge or certainty, the player must play the ball as it lies or, if the ball is not found, proceed under Rule 27-1.

Outside Agency
In match play, an “outside agency” is any agency other than either the player’s or opponent’s side,any caddie of either side, any ball played by either side at the hole being played or any equipment of either side. In stroke play, an outside agency is any agency other than the competitor’s side,any caddie of the side, any ball played by the side at the hole being played or any equipment of the side.
An outside agency includes a referee,a marker, an observer and a forecaddie. Neither wind nor water is an outside agency.