Ford has registered a patent with the U.S. Patent and Trademark Office (USPTO). This document details how such an invention would work.
Instead of projecting the owner's likeness against the window every time someone approaches, Ford envisions a facial detection system that would allow 'approved' users to walk up to the car without seeing the ghostly apparition.
If the person approaching the vehicle is not on the greenlist, the computer system would also have a means of analyzing the risk factor. This would be set in conjunction with a risk threshold, above which the car would deem the approaching person to be a threat.
When a threat is detected, the car follows multiple protocols, including ensuring the doors are locked and turning on the exterior lights. In some instances, these actions alone might convince a potential thief that someone is inside the car and is getting ready to drive away, acting as enough of a deterrent.
However, should the person continue approaching, the car would project the aforementioned image against the inside of the window. Provision is made for this being a made-up representation rather than just a photo, as Ford acknowledges the image of an adult man, for example, could be a greater deterrent than the face of a teenager.
The next step would be for the car to play warning audio through an external speaker. The patent describes multiple variations on the audio and the speaker itself, making representations for a vibrating element within the B-pillar to create a buzzing warning sound or alternatively using a genuine speaker that could play various recorded elements. These could include a pre-recorded 9-1-1 call simulation or a pre-recorded warning by the car's owner.
Source: Carbuzz