re: Has any OTer been accused by a rental car company of not returning a rental?

Yes. Forgot to leave the keys in the car and didn't realize until on the bus to the terminal. Dropped them in the provided "key drop" on the bus and assumed all was well. Rental car company called two days later to ask where the vehicle was. Was annoying, but we were able to resolve the issue over the next week or so. Only a minor PITA