Interesting Rule
Here is something that is often heard from amateur golfers, “I’ll go back and play a provisional, while you carry on searching for my ball”. Their hope is that, even if they have put a provisional ball in play from where they played their last stroke, someone will find their original ball within five minutes of search starting for it and they can therefore continue to play with it and abandon the provisional ball. Wrong! Part of Rule 27-2a states;
The player must inform his opponent in match play or his marker or a fellow-competitor in stroke play that he intends to play a provisional ball, and he must play it before he or his partner goes forward to search for the original ball.
So, once the player has dropped a ball at the point where they last played from (or played another ball from the teeing ground if it was their tee shot that is lost) that is now the ball in play. It cannot be a provisional ball, even if it is announced as such, and it is irrelevant whether or not the original ball is found, as it may not be played.