I haven't read the full T&C of "return when closed" lately but I remember something like: "Don't return the car when we're closed. If you do, there is a drop box for the keys and you are fully responsible for damages and time charges until we check the car back in." I've noticed this many times at non-huge airports (like BUF last week and MDT a few weeks ago). Sometimes the key drop is outside by the car return lanes and other times it's inside on the desk.

As for Avis.com (FAQ->USA->Rental Locations & Vehicles->Can I return a car to the location before/after business hours), they say "If you plan to return your car prior to or after business hours, please make specific arrangements by calling the location where are you are to pick up or drop off your car."

The only issue I've ever had dropping off a car during off hours was at WT9 (The Woodlands, TX, neighborhood location). They wanted to charge me for a windshield chip that was there when I picked up the car. I told them to pound sand because it was there (along with lots of other door dings) when I picked up the car and they didn't have me note that it had no glass damage upon checkout. They never called back.

As for the hours, my assumption is that they staff the location based upon first arrivals into the location because they cannot generate revenue (no ressies starting) at 430AM when someone is departing on a 545AM flight.