Question: Must the position of a ball be marked when taking a drop from an immovable obstruction, an abnormal ground condition or because the ball is considered unplayable?
Answer: - NO.
The position of a ball needs only to be marked when it is lifted under a Rule that requires it to be replaced. (E.g. Rule16-1b or 20-1). A ball to be dropped or placed in any other position such as when taking relief from an immovable obstruction, an abnormal ground condition or an unplayable lie does not need to be marked. It would however always be considered good practice to do so.