The level that shows a real-time mirror is Two Betrayals (and AOCTR if you wanna count it) in the Control Room floor. Now, in order to add one in Halo, here is how. First find any shader_environment (can't be a ground texture for a level, must be a different one). Then, open it up in Geurilla, and go down to Reflection Properties. Select Dynamic Mirror. Then, recompile your map again, and it'll show up as a mirror in game. You might get error messages, since this can only work sometimes. Warning, you must recompile your map like I said before, because the mirror effect won't come up without doing so.
