You would be looking for these words:

Mietwagen Rückgabe (Hire Car Return)

But at every airport I have returned to in Germany (and there have been a lot) - you can usually find everything in English or just follow the -Avis/Budget/Sixt signs... In most cases you will drive into a carpark (ground floor) take a ticket and follow the signs toward the company you rented the car from. There you will be met by someone who will direct you where to park - take the keys and wish you a safe trip home. So far I have never had a problem. Only hitch will be is if you have a Navi System that needs to be returned to the counter - then you will have to find the counter inside the airport.

All good. Cheers!