Each of the commonly used building entrances/exits (plus the front office) has a light board to indicate the status of personnel. Each light is matched to a person by surname: faculty at the top, then postdocs, then grad students, and finally a few staff members. The light boards are updated once a quarter by the office staff, so lights do sometimes move around.
There are two buttons below each red light. The upper button turns the light on; the lower button turns the light off. If you have a light, it should be turned on when you arrive and turned off when you leave. People often consult the nearest light board to determine whether someone is in the building, so please try to keep your light accurate. There aren't enough lights for everyone; usually people who aren't in CENPA much, or who don't keep their lights up to date, are the first to be removed from the light board.