Car rental charges me a damage I don’t know about

Hello! A little over a month ago I rented a small car – I had no accident and did not notice any damage to the car. I parked the car in the parking lot of the insurance and threw the key into the delivery box – so there was no personal handover. Today I got an invoice for about 600 euros due to an alleged long scratch – an consultation with me or similar had not taken place. Can you tell me, w he is the legal situation? Who has the obligation of proof? Who has a tip on how to behave best?