This is an enclosure for the common PIR sensor module, such as the one sold by Adafruit:
It has a 20 degree downtilt. 3 lids are available, one with no side/top blinders, one with 5mm blinders and one with 10mm blinders. The blinders could be useful to avoid detection from areas other than directly ahead and below. A common use case for this would be to detect a person as they approach a door and trigger an electronic release.