Damage to the car

I rented a car from Advantage Rent A Car in December 2011.  I returned the car on December 12, 2011 with no damage.  I received a call from someone from Pur Co Fleet Services stating that I had damage to the car I returned.  On the day that I returned the car that I rented, I left it in the Advantage return parking lot for about 20 minutes so that I could rent another car from Advantage on that same day.  In addition, I had to unload my things from the rented car that I had returned so I could put them in the car I had just rented.  This took me another 20 minutes.  I walked back and forth from one car to the other while unloading.  Neither I nor the attendents who allowed the car to remain in the return section while I rented another car saw any damage on the car I returned.  When the person from Fleet Services Called, he could not tell me the date the damage was done nor could he give me any substantial information.  Everytime i tried to explain to him that it was not possible for me to bring back a damaged car without any one noticing it, he proceeded to ask me how did I want to pay for the damage. I would like to know what I need to do to rectify this situation.  There is no way that I could have caused the damage to the rear bumper and not notice.