Is there an alternative to radio waves that can go through metallic objects? Radar can pass through materials such as paper, wood, glass, brick, and concrete, but it reflects off of metal. Is there an alternative to radar that can pass through metal substances? If not, is it likely that we will ever find such a wave?
 A: Neutrinos and gravitational waves should be able to pass through most substances - the problem is that they won't be good for sensing any of the objects through which they can pass.
Sound waves can travel through metal, and they can be detected by things such as microphones, so perhaps that is what you are looking for in your radar-like device.
A: 
Radar can pass through materials such as paper, wood, glass, brick, and concrete, but it reflects off of metal.

One wants radar to reflect off metal. That way, metalic objects can be picked up by radar. If the microwaves in radar passed through an airplane, or a boat, or a missile, without being reflected, then they would not be of much use in detecting those objects.
There are other waves that pass through objects, including metallic objects. These include gamma rays. Besides potentially causing cancer in those who might be exposed, they would be of little use as radar for the reason given above.
Sound or ultra-sound might be an option for detecting objects hidden behind other metallic objects, provided the distance is not too great.
A: As pointed out in another answer, ultrasound is an option for detection of objects behind sheet metal. Actually developing such a system is very difficult, of course.

The rear doors of the Tesla model X are wing doors (referred to by Tesla as 'falcon wing doors'). These wing doors are moved by electric motors, controlled via software interface. That means the doors have to be equipped with sensors to detect whether some obstacle is present. Elon Musk, the CEO of Tesla, describes that he was keen to avoid to have a detector/sensor visible on the outside of the door panel; he was keen to achieve a very clean look.
The Tesla engineers succeeded in developing ultrasonic sensing that operates through the aluminum door panel.
As attested in video footage: the model Y wing doors detect the presence of any obstacle, and avoid touching the detected obstacle when the user gives a command to open the doors.
Examples of what counts as obstacle: a person standing next to the door, another car, a low ceiling of the garage.
A: For imaging, one alternative is neutrons. Neutrons tend to pass through heavy elements and are absorbed by light elements, so they can be used to image light elements that are present inside or behind heavy elements. You can see an example image here.
One disadvantage is that neutron radiation is harmful to living things and requires a nuclear reactor or a particle accelerator to create, so it's not practical for all uses.
