After hours returns are those in which you return the rental car after the location is closed, normally by using some sort of key drop box or key return kiosk.
More and more, companies are restricting this practice, unless they are a large airport location that can have a drop box in an area that is still patrolled by security.
For small neighborhood locations there are liablity issues. Because they can't inspect the vehicle when it's returned, the rental car location must assume that any damage to the vehicle is the responsibility of the renter.
It's almost impossible to prove that when you dropped off the car, it was fine! (Unless you take photographs of the vehicle with a camera that is date/time stamped that can't be manipulated.)
Even if a car rental location does not generally offer late drop off, you may be able to work out an arrangement.
Your best bet is to call the location and ask to speak to the manager. They may be able to work out something with you, that will allow you to return the vehicle after the location has closed.
Make sure that you have a clear understanding of what the rental company determines will be the drop off time. If they will not officially check in the car until they open in the morning, then your after hours return could cost you an extra daily charge anyway.
If you are not returning at the time scheduled on your contract, you should check your provider's policies for extending your rental beyond the date and time specified on your rental agreement.
Most airport car rental locations that are not open 24 hours will still allow late drop offs after the counter closes. There is a lot of security on the grounds of the airports and in the parking garages. Each location will determine the availibility of this option. Check with the company directly for restrictions and instructions on how and where to drop off the car.