In petanque, there are only a few things that can legally (note: legally) move a ball in a game. It can be thrown by a player. It can fall onto the terrain and bounce or roll. It can hit or be hit by another ball in the game. And that’s about it.
By "ball", I mean a boule or jack. By "move a ball" I mean change the ball's location or speed or direction of movement... to "displace, stop, or deviate" it.
There are a lot of non-legal things that can move a ball. The wind. The shoe of any human being. A dead boule bouncing back onto the terrain. A child or an animal or a football crossing the terrain. Being thrown “contrary to the rules”.
In all such non-legal events, the default course of action is simply to leave things where they are and carry on with the game.
Muttering under your breath is therapeutic, but optional
The FIPJP rules repeatedly mention another course of action— to put the ball back in its original location if that original location was marked. Some players take the repeated references to marking the balls as an indication that they should always mark the locations of everything. And they claim that, where they play, that’s what everybody does.
That’s nonsense. First of all, in normal play nobody does it. In all of Youtube, you will not find a single petanque video filmed in the last 20 years in which the players routinely mark even the jack. (At one time, it was traditional for the team that threw the jack to immediately mark the jack’s location. That way, if the first boule pushed the jack and the opponents challenged the jack, the original location of the jack could be established. But that custom faded long ago.)
Second, marking everything is a terrible idea. There were enough problems with circles drawn on the ground, back before plastic circles appeared in 2005. In a triples game of 10 menes, we would need to draw only 10 circles. But we would need to mark 130 initial locations of jack and boules, plus perhaps 40 more locations for balls that are moved. There would be so many marks on the ground that if you wanted to return a moved boule to its original location, you wouldn’t be able to find that location amidst the welter of marks on the ground. To control the mess, you’d need to sweep the terrain after every mene.
There are a few situations in which the wise player will mark the location of the jack. On a windy day, you should mark the jack in case it might be moved by the wind. Or if another game moves into a position where its boules might come onto your terrain, you might want to mark the locations of your balls. But it’s certainly not something you want to do all the time.
So if you’re out playing with your friends, and nothing is marked, and a ball is moved in some non-legal fashion, don’t worry. You know what the standard, officially-approved FIPJP course of action is. Just leave everything where it is and carry on.
If you’re with friends, and you know where the original position was (even though it wasn’t marked), and everybody is cool, you can put it back. But remember that in an organized, umpired tournament, if the umpire comes over and he doesn’t see marks on the ground… everything stays where it is.