How does Google Maps Navigation know you are in a tunnel when it switches to night mode? I suppose this isn't exactly a new feature but it is new to me since I don't travel through tunnels very often.  What exactly is the software using when it decides to switch over to night mode when in a tunnel?  GPS Location? A light sensor? A combination of both?
Hard to say without knowing which device and firmware version we're talking about, so what follows is my speculation.

I have a Garmin Nuvi which continues navigating in a tunnel, turns and all, despite losing its satellite reception.  Mine doesn't switch to night mode, but it clearly knows it is in a tunnel from the map data. Since loss of reception is a given in a tunnel, the device must be dead-reckoning while it's in there. (Re-broadcasting is out because it would affect the signal timing which is key to calculating a fix).  In fact, I have tested this by entering a tunnel fast and decelerating once inside it.  The device alerted that it had lost reception and continued to display my entry speed, "passing" turnouts and the tunnel exit before I did.

From that, I inferred that the firmware was using map information, possibly cross-checked with loss of satellite lock, to know when I've entered a tunnel.