Earlier quoted context omitted.
German law states it can be used as a signal before passing. Nobody does it though, which is a good thing.
Sure about that? You can use your lights to indicate that you're planning to take over, but the horn? Never heard about that one. Interesting.
"§5 Passing (5) Outside of cities/villages it is allowed to notify of a planned passing by short sound or light indicators. " (Uff, legalese is hard to translate.)