Even the most charitable of golfers will get his dander up seeing someone or something move his golf ball without permission. Territorial lot, we are. But what happens when that squirrel runs away with your ball, or that codger on No. 16 puts it in his pocket?
Rule 18-1 says that if a ball is moved by an outside agency (anyone or anything that isn't involved in your round, caddies excluded), the ball must be replaced at the spot where it was moved, without penalty. If the ball can't be found, but you know it was taken by an outside agency, you must put a different one in play.
You must also put your ball back in its original resting spot if it's struck by another ball.
If your ball is moved by a fellow-competitor or his caddie during stroke play, there's no penalty. If the ball is moved in match play, other than in searching for it, then the side that moved it receives a one-shot penalty. This includes a ball that's moved by an opponent who thinks your ball is in the way of his next stroke, even on the green. He needs your permission to move it.
What if a ball is moved, but you can't determine where it was moved from? Using Rule 20-3c, through the green (anywhere on the course except hazards, and the green and teeing area of the hole being played), the ball must be dropped as near as possible to the place where it lay. Same goes if it was in a hazard. But it must be dropped in that hazard. On a green, it must be placed, not dropped.
You probably know that, except on a green, you can't lift or cause your ball to move while removing loose impediments (see leaf, above). It's a one-stroke penalty if you do, and the ball must be replaced (Rule 18-2a). However, if your ball comes to rest next to or on a movable obstruction (see tinfoil, top), you can remove the item. If the ball moves, you must replace it without penalty (Rule 24-1).
Q: Your shot lands in a pond on a par 3. You hit another just short of the green only to discover the hole has a drop area. Can you forget about your second tee shot and play from the drop area?
A: No. Not being aware of the drop area doesn't entitle you to a do-over. Always check before hitting another ball from the tee.