What is the right course of action now?

In Germany, the "Rechtsfahrgebot" (keep-right rule) requires drivers to use the right lane whenever possible and only use the left lane for overtaking. After you have safely overtaken a slower vehicle, you should return to the right lane to allow other vehicles to pass if needed and to keep traffic flowing smoothly. Staying in the middle lane after passing is generally not allowed unless there is a specific reason, such as heavy traffic or road signs indicating lane use. So, after overtaking, moving back into the right lane is the proper and legally required action, ensuring safer and more efficient driving on multi-lane roads, especially highways.