Seems like a very strange dropoff to me. Every dropoff I've ever done either I meet with a live person to whom I hand my keys (or leave them in the ignition since they'll need to put them in the ignition to check the return mileage, the gas guage, etc), or if it's unattended there's an envelope where I'm supposed enclose my keys in a key drop.

If this was an unattended return, what did they instruct you to do with the keys?

If there was a live person, why didn't they take possession of the keys while you were still there?