After returning the car in Munich and boarding the train to Frankfurt, I realized that I hadn't returned the keys. I proactively called to ask if I should send them back via DHL from Frankfurt or return them to Europcar at Frankfurt Airport. After a while, the branch called back, insisting that I must come back immediately and take full responsibility for any consequences. I explained that I had an international flight connection and couldn't just jump off the train. They kept insisting that I return immediately or face huge fines.