Communicating with the rental agency is probably not necessary because there would be language in the rental contract that would outline what the actions would be if the car was returned late. Being charged with grand theft auto was probably not in the contract.

If I loaned a car to a friend and he kept it a few extra days I hardly think that the cops would see that as GTA. With the rental agency, they even have less to stand on because extra days would be outlined in the contract. It is likely that the rental agency and the cops were operating outside the contract.