Most rental car companies will allow you to return your vehicle after business hours. However, you will need to pre-arrange this with the depot directly, and you will need to get familiar with their specific policies.
Airport locations usually allow after hours. But it is recommended you check with the rental location when picking up your rental vehicle for exact procedure. Please note that vehicles should be returned to the same location where picked up unless prior arrangements have been made with the original renting location. In addition, if you drop off your rental vehicle at the airport but did not originally pick up your vehicle at the airport, an additional drop fee may be charged.
For non-airport locations, they require that their rental vehicles be returned during normal business hours. If you want to return a vehicle when the branch is closed you must check with them when picking up your vehicle to determine if an "after hours" service drop box is available. Doing so will help you avoid additional charges. Please note that any vehicle that is returned after hours will not be checked in until the next business day. If returning after hours when a branch is closed, the customer is responsible for the vehicle until the satisfactory check in of the rental vehicle the next business day.
Read more about returning your rental car.