Official Golf Rules on Moving Golf Balls at Rest
Referees, spectators and forecaddies are considered outside agencies. If a ball at rest is moved by an outside agency, then the ball must be replaced. The player incurs no penalty.
By Player, Partner, Caddie or Equipment
If the player, his partner, caddie or equipment accidentally causes his ball to move, the player incurs a one-stroke penalty. The ball must be replaced to its original position. The only time a player does not incur a penalty is when searching for a ball in sand, abnormal ground conditions or water. In those cases, if the player accidentally causes the ball to move, it must be replaced with no penalty.
Moved at Address
If a player sets his club behind the ball and his ball moves, the player is deemed to have moved the ball. It is a one-stroke penalty and the ball must be replaced. The only time the ball is not replaced is when the ball moves during the player's swing and the ball is hit. A one-stroke penalty is still enforced.
By Opponent, Caddie or Equipment
If a player's ball is moved by an opponent, his caddie or equipment, the opponent incurs a one-stroke penalty and the ball must be replaced. If it occurs during a search for the ball, there is no penalty and the ball is replaced.
Ball in Motion
If a ball at rest is moved by another ball in motion, the ball at rest must be replaced without penalty.