Always record and take pictures of rental vehicles upon pick-up and return. Fortunately, I did. SIXT in Barcelona, Spain decided to email me about damages I did not do because I inspected the vehicle and returned it in the condition I picked it up. I took pictures, and did a full video recording for my records. They send emails about damage claims to the vehicle and ask for payment for the damages. Plus, most importantly, there was no Human Being to inspect and check the vehicle upon returning it (like other rental agencies do). -AVOID THEM AT ALL COSTS! These are predatory actions they conduct and share your concerns with as many media outlets as possible to get the word out about their unethical practices. Their protocol is to park the car and drop the keys in a box in a parking garage with no Human Inspecting the vehicle to close out the rental contract.
Recommendation: Avoid renting from SIXT (use Hertz or Enterprise who practice conventional means to inspect a vehicle upon return)