I've read about this a while back... actually, once you know the trick, it is pretty easy to do... the trick is that the robot first determines the make and model and year of the car (perhaps by reading a bar code or rf chip that all the auto makers agree to put on the vehnicles). Once the robot detects this info, it uses simple vision processing to look for a distinguishing feature in a specified area (perhaps a door handle on the rear part of the car, or something). Once it locates the distinguishing feature, it then looks up in it's data bank where the fuel door is (in relationship to the distinguishing feature), what processes to use to open it, etc., etc. etc.
Also, parts of the film where in "fast forward" to give the illusion of the robot moving much faster than it acutally can.