If your story is in fact just as you describe it there is no way possible that you will held accountable for anything for several reasons. Don't have the time to list all of them but they are fairly obvious.

Even if the damage was extensive if you were not notified of any damage when you returned and paid for the car it is impossible for ERAC to hold you accountable for the damage b/c they can't prove when it happened. It could have happened after you dropped of the car and put into ERAC's hands.
Once you return the car to ERAC and turn over the keys and car it is their responsibility.