Claims that I damaged the rental car

This is following the complaint that I made about the Avis billing account without my consent on the 26th Jan 2016.Now they are making claims that I have damaged the car and that I owe the R3 850 for the damages.The damages claimed there are for radiators,and front bumper and other damages.My main concern is that when we returned/dropped off the car,it was already late and the offices were already closed and this actually was insisted that we needed to drop off the car even if the offices were closed we can leave it with the guard.This car was checked the following day whilst we were not even present and my concern is we don't even know what transpired after we dropped off the car with the guards.I will certainly not take responsibility on the damages on this car until Avis fully investigates what happens.When we returned this car there was no damage to the car and therefore will not be able to pay this claimed amount.Avis Menlyn is just passing the buck on me also because of the previous complaint report I did.