Retaining deposit

Yesterday i rented a car fron Europcar. When i returned it today a hubcap was missing. I pointed out to the guy collecting the car that there were no marks/scratches on the tyre or rim indicating a pavement had been touched or that a collision had taken place. He agreed and said it must have just fallen off while driving. I asked whe they did not secure the hubcaps using cable ties and he said his boss says it looks unsightly. Now a claim has been instituted against me for the loss of the hubcap. Surely it is not my responsibility if a hubcap just falls off?? Again an example of the big companies milking the consumer and taking no responsibility!