I figured it out. It lights up when the battery is charging. It goes out when the battery is charged or when the power supply is unplugged. I wonder why it's not mentioned in any documentation.
I just bought a Dell Refurbished Precision 5530, and there's a light on the front edge, lined up with the center of the trackpad. I've read all the available documentation but can't find any reference to it. It's not associated with the keyboard backlight, so it's not that leaking out. It looks like a clear lens is part of the black palmrest, just above the sliver edge of the bottom cover. It's not always on, so it appears to be indicating something. Does anyone know what this light is, and what it's indicating?
Precision 5530 odd light