It sounds like they pulled a fast one on you. The agent probably stuck the LDW on there knowing that you wouldn't see the rental contract so no way to argue about it (rental agents are commissioned salespeople). You could contact Sixt on the basis that you a) never saw a rental contract and b) never signed anything. Not sure you'd be successful, but it's worth a shot.

PSA -- It's extremely important that you have a copy of the rental contract before you leave the rental office, both to verify what you are being charged and in case of an accident or if you get pulled over by the police. If you had gotten pulled over during the rental period, you had nothing at all to show that you rented the car. Given all the problems with Hertz of late with folks getting arrested for driving (improperly reported) stolen vehicles, this is a very dangerous spot to put yourself in.