If I ever have to drop off a rental like this I'll break out my iPhone, turn on the video camera and take a mini movie of locking the doors, checking the doors, keeping the keys in sight, do a slow walk around the car showing condition and then walking over to the drop box and put the keys in.
And I know one thing: I will never rent a car from Enterprise again. I generally prefer to avoid them, but now they are just verboten.