This is a Curtain Motion Detector. A curtain motion detector is used to detect someone walking across a path. This is much different than a normal motion detector that would be used in a burglar alarm system since those types of motion detectors have a very large field of view.

The motion detector can be surface mounted to a surface such as a drywall wall or ceiling. There is also a swivel mount which allows the device to be mounted and aimed if desired. This device can sense people and animals. 

Two of these curtain motion detectors are included with the stair lighting controller model 2000 but they can also be used for many other purposes.

The motion sensors are 11cm tall, 6.6cm wide and 4cm thick.

There are jumpers that can be set on the inside which allow this device to be set as a N/O (normally open) or N/C (normally closed) style.