Keeping Refund : Blame for prior damage on vehicle

When i went to collect the vehicle we hired on Friday 1st December 2017, the arranged pick up time was 10h00, we had to wait 1hour because the vehicle was not ready. The vehicle was inspected by myself and the agent, but not underneath the vehicle. As it is not our job or trade we were not aware of all the checks and being a agent you know what checks is needed, so we re**** on the agent to do it correctly which was not done. When we returned the vehicle on Sunday 3rde December 2017 at 18h30 the agent at the airport office did not check the vehicle with me, he only took the keys from me. I received a call from Thrifty on 4th December after 19h00 informing me that the vehicle is damaged and that they will not refund my deposit. When we went to their office on the 5th December to enquire about the damages, they indicated that the damages were done underneath the front bumper. We did not damage this vehicle, because we would know, and that we returned the vehicle in the same condition it was given to us. They assured that there were no damages when we were given the vehicle. We asked for the tracker to prove to them that the damages were not caused by us. We informed them that we were instructed to only leave vehicle on Sunday without agent checking, and we informed them that the agent om Friday did not check underneath. We are the customer and we are not aware of all the checks. They just said that they will get 3 quotes and that we are still liable, even with the knowledge that no proper checks were done. I feel this is unfair and that we are honest people that anything could of happend prior or after vehicle were handed back, because they contacted me more than 24hours later.