This is pure speculation, but I wonder if google has photoelectric sensors that use light levels to send alerts to their administrators.
I bet it's a clever way of handling physical security. If anybody is accessing the server, they'll want a flashlight. Light sensor detects the light and alerts an admin. If it isn't a scheduled access, then danger will robinson!
Or similarly, perhaps they want anyone lingering/lighting their areas to be extra-obvious on security footage.
Without an explanation like these, it's hard to understand the security-gradient provided by darkness, against people who have leisurely physical access. Spies whose access isn't being closely observed can bring their own lighting, cameras, and even infrared-equipment, too.
I bet it's a clever way of handling physical security. If anybody is accessing the server, they'll want a flashlight. Light sensor detects the light and alerts an admin. If it isn't a scheduled access, then danger will robinson!