Thrifty adds to my bill 24 hours later

I made sure I filled up our hired car en route to the airport where I returned it. The woman receiving the car at O.R. Thambo Airport turned on the ignition and was happy the tank was full. <br> One day later - with no warning or notice - I see Thrifty have deducted. R62.34 off my bill. I phoned and queried this and they say they had to fill the tank. I immediately disputed this and they promised they would get back to me. <br> I'm still waiting.<br> What I don't understand is how they can agree they are happy the tank was full and then change their minds and decide the car wasn't full?<br> How do I know someone didn't use the car or syphon petrol from the car after I left it behind?