Unlike some other rental firms, SIXT employees at DUS will wait even if your delayed flight means you arrive after official closing hours. And they were courteous and cheerful even though it was past their working hours (Danke, Frau Coban!). I wish though SIXT would made this information more easily accessible and have the "speak to a human on a phone" option in their automated phone system (hello to all stressed out travellers, may you find this on Google).