In my BMW, when I go out to dinner at a restaurant that has valet parking, I put my briefcase or other valuables in the trunk, flip the trunk lock switch in the glovebox, lock the glovebox and give the valet the fob only. You can't fold down the seats from the cabin side of the seats, only from the trunk side; so the trunk is secure. In my previous Audi, the seatbacks actually had key locks on them so you could accomplish the same.
Does the ATS offer a way to do this? (my car's on order).