Among the different types of sensors used in cars, some of them monitors the outer condition of the vehicle:

BSD: Blind Spot Detection

The BSD millimeter-wave radar is activated once cars in adjacent lanes go into the blind zone.

FCW: Forward Collision Warning

The FCW is activated once the vehicle comes too close to a vehicle in front of it.

RCTA: Rear Cross Traffic Alert

The RCTA is activated when the car is in reverse mode to detect cars, pedestrians or other objects behind it.

DOW: Door Open Warning
The DOW warns drivers of any fast approaching vehicle or person when they open the door.

LCA: Lane Change Alert
The LCA is activated when the driver tries to change lanes when other vehicles are detected in the blind zone.