How do you lock the lift gate??

When you lock all of the doors, the liftgate will be locked as well. If it opens even after locking try locking the doors, leaving the key away from the vehicle, and then go back and try to open it. It should not open then. The entry systems on most newer luxury vehicles allows you to open the door without pressing unlock. This is accomplished by the vehicle sensing the key is nearby. Did the test work out?