all return sites have key drop boxes , I've used them after hours on numerous occasions.
They have a drop box for keys for after hours returns. You will notice on your rental agreement that you are still responsible for anything that may happen to the car while it is on their lot until their doors open the next business day. I have never had any issues but you do need to be aware of this policy.
They are open Sundays unless it's a holiday I believe. With that said, you need to plan your returns according to the return locations schedule. Never hurts to call and ask what your options are.
Airport Enterprise is always open..
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