I hadn't realized this before, but yeah, it's the path that's visible, with or without waypoints. You can easily replicate this using the new Open Player View feature just added, although I also replicated it using a separate test user account. In the ruler, it's the Broadcast to Others setting that does it. Turn that off, and the player can't see anything. I'd agree that it showing through darkness feels like a bug, and I suggest reporting it.