Many large areas are not used very often, and lights may be left on unnecessarily. These low-use areas include mechanical rooms, boiler rooms, pipe basements or crawl spaces, attics, and storage rooms.
An inexpensive pilot light mounted above the access door alerts staff when the lights are left on and saves the expense of operating the lights for long periods of time when the space is not in use.
A more sophisticated solution to this problem is to install occupancy sensors to disable the lights when the space is not in use. Infrared sensors work best in rooms with a high noise level such as mechanical rooms, while ultrasound devices are ideal for most other areas.
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Excerpt from Energy Conservation Projects.