I reckon that mainly applies to those hire car operators who run their places outside of the airport.The ones inside usually are open virtually 24/7 and even then the operating hours are usually clearly shown on their website.
If its a drop off its dead easy you just post the keys in a box somewhere.
It's the local operators who tend not to. The best bet is to find the big shared desks for the mainstream operators who 'code share' out of normal working hours. Or one with an automated pick-up machine.
