This is a suggestion borrowed from the latest Ford Escape, where if you have the keyfob in your posession, you can go up to the trunk, wave your foot somewhere under the rear-bumper, and it opens. the theory being if you have your hands full of shopping, you don't have to put stuff down to find the key or press a button. Although, honestly, I'm not sure this a a feature that would get used much, it does have that 'cool' factor, and Ford are certainly using it in their marketing. For the Model S, since there's an HD camera and motorized truck opening (with tech package), I figured that it would be able to sense the key, and 'see' that you're stood at the back of the car, so would pop the trunk if you stood there, still, for more than a second. I'm assuming there's a dedicated sensor for the rear of the car, I beleive that's the 'norm' for keyless-entry cars.