Just to give an update - Avis emailed me a questionnaire a few days later: What kind of car was it, When/Where did you pick it up, When/Where did you drop it off, was the location open, where did you leave the keys, did you re-fuel the vehicle, etc.

I replied, and Avis investigated. They sent me a bill a few days later that said that I only drove 2 miles, and returned it full at exactly the times I had reserved it for despite telling them otherwise by email. I'm guessing that they never recorded the fact that I actually picked up the car, and that if I hadn't called I would have never been charged.