While searching for a friend’s ball recently I found one of the same make and called him over to identify it. He went straight in and lifted it up to see if his mark was on it before letting me know that it wasn’t his ball. My reaction was to say that “I was glad that it wasn’t as he would have immediately incurred a 1 stroke penalty if it had been, because he had not marked the position of the ball.”
30 seconds later, we found another ball, and he came over to identify it again. Without thinking, he immediately lifted it without marking again! However, this time it was his ball, so he incurred a needless 1 stroke penalty.
If you want to look it up, its Rule 7.3 which covers this situation very clearly.
Remember – Whenever you lift a ball that is to be REPLACED (i.e. in the exact same spot) you MUST MARK its position. It is good practice to always mark the spot of a ball before you lift it, even if you do not need to replace it on its exact spot.