This happened to me too - last Saturday I dropped off a car before opening hours using the dropbox. I filled it up on the way to the rental location - needle was well above F. Called the rental location after they opened to check that everything was good - the agent confirmed the total charges I expected and said everything was set. Then, email with "Hertz statement of charges" came through yesterday with an additional "fuel and service" charge added - for only $19.77 (compared to $32), but still...

Called Hertz - the rep immediately said she "would remove the charge and log a complaint" (her words not mine, and very quick and smooth - it didn't seem to be the first time she had fielded calls about erroneous fuel charges). Yeah, quite a number of people might fail to notice an extra charge and call in about it - this would definitely add up. Another thing to check in the future no matter which rental company is used.