It's kind of offsite, but a only a 5 min walk back to the airport. All the car hire return is in one area. We've done the early flight back, and I'm sure we were able to drop the car earlier than that - or if there is more than one of you, perhaps someone could go to check in while someone drops off the car?
The other biggie at TLS is that at that time of the morning, the only petrol station near the airport takes French cards only - we got caught very badly with this

Fill up before you get to the airport!