Enterprise Rental Car damage recovery claim South San Francisco, California

Had very good experiences with Enterprise in the past.  Good price and service.  Rented a car for 2 days while on vacation from this location and at the time I rented the agent noted a very small dent in the fender (I might not have noticed) to me and on the form which I signed.  The walk around was very thorough.  I also purchased their additional insurance.  Returned car 2 days later without and issues.  The agent did a check-in walk around, no problems.  Paid my bill and they drove the car off for another renter.

About a month later I received 2 not so nice letters stating I had damaged their car and they included a bill. They wanted immediate payment and noted that failure to respond in a timely manner will void any coverage I may have.  No questions, no request for information, just 2 letters stating I damaged their car and demanding prompt payment.  There were at least 2 problems.  One is that I did not damage their car, the other is that even if I returned the car with damage (which I did not), I had paid for the insurance.  The damage they were referring to was the small dent first pointed out before I rented the car, and noted by their agent.

Upon calling the "Damage recovery unit" it seemed that they did not have any backup information about why they thought it was me, and had no information from the actual rental location including the fact I did purchase insurance.  Upon stating the facts to them they backed off quickly and closed the case.  I followed up the location in South San Francisco who notified me that their notes on my rental did not indicate any damage or sending in a claim.

Even though this appears resolved, I am very disappointed and questions remain.  Why does the "Damage Recovery Unit" not have information form the rental location, why did they decided to falsely accuse me, why did they attempt to recovery money from me after I had purchased their own insurance, and did they try to recover this money from other unfortunate renters of that same car?

In the future I learned, and I recommend this to anyone renting from any company, take pictures of the car from as many angles as you can and save all your paperwork.