So, you don't actually know where the sensor is or how to adjust it.
The good thing about your comment is that you did inspire me to search and test rather than just gripe that I should be able to turn off that useless feature.
There is a cigarette-carton-size plastic box mounted on the topside of the flimsy rear facia on the passenger side. That must be the 99-cent motion sensor. There doesn't appear to be any kind of adjustment pot on it. I'm certain that the sensitivity is adjusted via software. The box is just clipped to the top of a long, thin, plastic facia with no support for several feet. I expect something mounted to the frame, not to the top of a piece of plastic about a 1/16" thick with no support anywhere around it. One of the things that I learned long ago was motion and vibration sensors need to be mounted RIGIDLY so that there is no motion of the sensor that would be interpreted as motion of the object being sensed. Apparently Ford doesn't understand this. So, anything that causes the rear facia to quiver will cause enough movement to set off the sensor.
Then I discovered two "undocumented features" of the system:
1) Rap the rear facia anywhere and the whole thing quivers causing the motion sensor to open or close the rear hatch. The rear facia is so flimsy that if you bump it (either side of the hitch), while loading or unloading the vehicle the resultant quiver will cause the the rear hatch to open or close. Drop a bag of cement in the back and the quivering will cause the hatch to close (at least on a very hot day when the plastic is particularly soft). That's all it takes, no foot motion, no body motion, simply bump against the rear of the vehicle or drop something that causes a sharp impact and the hatch will automatically open or close.
2) Motion sensor activation DOES NOT result in a warning chime; when the motion sensor activates the rear hatch there is absolute silence. The chime DOES work if you press any of the buttons to open or close the hatch. Interesting that the chime, which one would expect would go off as a warning in unintentional activation, only goes off when you intentionally activate the rear hatch. Sounds backwards to me.
My guess is that I could go out and buy a 36" long piece of angle aluminum and rivet is to the top side of the rear facia and connect that to the frame of the vehicle to provide the needed rigidity. But why should I have to do that to a new $50,000+ vehicle? My only alternative is to take it in to FORD to have them adjust the sensitivity of the sensor via a software tweak and fix whatever is wrong in the programming that doesn't causes the chime to sound during ANY motion. But, then, why not just give me the ability to disable the maddening feature with a menu item that would set the sensitivity so high that the motion sensor won't activate?