If you sign up for the full insurance including 0 deductible you won't ever have any issues. I usually not even check the car at pickup for any undocumented damage and with around 20 rentals in the last 2 years in different European countries never had any issues.

Just once... where I rented for a private trip and saved on the 0 deductible and opted for the 300€ deductible they wanted to charge me for damage which was already there when I picked up the car. The damage report said "scratch of 10cm on the front bumper" while the actual damage was easily 35cm. I showed the damage to the garage guy. He told me that the damage is noted and the Sixt system only knows four states: Undamaged, scratch under 5cm, 5-10cm and wrecked". Since the bumper was not wrecked and needed immediate replacement they put it down as 10cm damage.

Returning the car the return agent gave me hell and told me the scratch is significant larger then 10cm... Luckily I did photos during pickup and noted the name of the guy in the garage. They dropped the charge but it left a pretty bad feeling they tried to scam me since I dared not to pay up for the full insurance.