Damage Claim for dent

I hired a Hyudia i10 with Avis at Cape Town. I was told it has a front cap hub dent that I should be aware of. I checked the car, all looked fine. So I drove off. Upon returning the car there were four men waiting who took a look at the car. I told them what issue they said the car has, they could not see it either and gave the word that the car was well treated. Negligently I left without even a signed document to say that the car was returned in good condition. A day later a different inspector check the car and calls to say that the car has a dent. I am sent a damage form and images of a dent which I cannot see. A full day later, anything could have happened between the time I returned the car and the following day. It is ********* to call me a day later about a dent which I was not made aware of when returning the car. I cannot be held liable. Avis Cape Town International Airport business practices are wrong. The worst part is that the inspector who contacts me only identified that dent with the car, and the one I was told is there he could not identify either. Dissatisfied customer!