Sensor Physics

Static and ghost.
Invisible by default.
Always active to ensure correct collision detection.
Capable of detecting both static and dynamic objects.
Ignoring collision with their parent.
Automatically removed from the simulation when no collision sensor is active on it.Unlike the Near and Radar object it can:
Be made visible for debugging (just use the Visible actuator).
Have multiple collision sensors using it.Other than that, the sensor objects are ordinary objects.You can move them freely or parent them. When parented to a dynamic object,they can provide advanced collision control to this object.

The type of collision capability depends on the shape:

Box, sphere, cylinder, cone, convex hull provide volume detection.
Always prefer simple shape (box, sphere) to complex shape whenever possible.
Always use broadphase filtering (avoid collision sensor with empty property/material).
Use collision sensor only when you need them. When no collision sensor is active on the sensor object,it is removed from the simulation and consume no CPU.Known limitations
When running Blender in debug mode, you will see one warning line of the console:

Collision Bounds