At the end of the rental, you should return the car where you and the owner agreed to do the check out.
For Open rentals, you need to give the car back in the same area where you took it, meaning within a radius of 400 meters / 5 minutes by foot. If you collected it in a private parking, you simply have to return it at the same place. It’s best to see in advance with the owner what are the accepted areas.
If you can't bring the car back to the owner, a fee of 100€ will be applied.