National tried to do that to me on a rental in the US. I called them and told them I did not do the damage. They tried to argue that someone may have hit the car during my rental period but I was in the car for every minute of the rental so they conceded my position and stopped the charges. I'd give them a call. It could be that they have a corporate policy of blaming unattributed damages to customers in the hope of getting some $$ and then dropping the charges if the customer complains.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13