Marking and identifying your golf ball are of the utmost importance when playing a round of golf. The reason why so many people are sticklers about their marks is because when playing under the Rules of Golf, you incur a two-stroke penalty should you accidentally play the wrong golf ball.
It doesn’t matter if it’s a range ball, your opponent’s ball or a random ball lost by another player that day; the penalty is two shots and you are required to correct your mistake by locating and playing your ball from its original position or proceed under the Rules should you have any further issues.
If you were to play a fellow competitor’s ball, there is no penalty for the other player. They simply have to return the ball to its original position and play in from there.
You are allowed to pick up and identify your golf ball under the new Rules of Golf without informing your opponents or playing partners, but you must replace the ball into its original lie and state after IDing it.