[darklajid]

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.

[kaybe]

"§5 StVO Überholen (5) Außerhalb geschlossener Ortschaften darf das Überholen durch kurze Schall- oder Leuchtzeichen angekündigt werden. .."

"§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.)