Its standard on S2 sedans as far as I am aware. Its actually called the Intelligent key. If you have the chance to check out a Series 2 seda, have a look on the drivers side door and also the boot, there is a button there. If you have the key fob in your pocket, you just press the button to lock/unlock doors and bootlid. Same thing when you are inside the car, as long as you have the key in your pocket or close proximity, you just turn this knob next to the steering wheel where you would normally insert your key (keyless start).
Check out this video clip: