One of the new technologies that Cadillac is bringing to their line in the new XTS is capless fuel filling.
In this implementation, there is no removable fuel cap. Instead there is a build in flap that seals the fuel neck except when a fuel nozzle is inserted. The advantages include a smaller fuel door as well as eliminating the long-standing issue where a cap is not put back on or not tightened sufficiently and results in a check-engine light (because a lack of a sealed fuel system is an emissions problem that triggers the light).
Now, if Cadillac could only lick the ‘I forgot to close the door’ problem next.