Detecting something and protecting from something are different things, as Cf498 said in his comment, they just go directly to the protect step and ignore the detect step. They don't have time for Detect step. Its just that in the way he phrased his first comment, it was unclear, but I didn't assume that he was actually confusing the 2 steps, that's why I used "if" and said that I didn't understand clearly. Actually he didn't needed the info in that answer, but some others readers could get confused, as it seems to be my case. Now if you ask me how could I possibly think that cover a mirror is a way to detect a two way mirror, then I have to say I don't know, but maybe I imagined that if its day time, and the curtains don't provide enough darkness, you could cover the mirror with your head inside the cover to provide enough obscurity for your eyes to see trough the two way mirror. So maybe my imagination went too far, but yeah.