This can be a problem when renting from non-airport locations, not all of which have on-site gas pumps. But this should not happen at an airport location.

Note that some renters pre-pay for a tank of fuel. It's in their interest to return the car as close to empty as possible.

These days, it's important to compare the fuel level and odometer reading inside the car with what is printed on the contract (as well as checking the expiration dates of the registration and inspection stickers).