One of the joys of Cabin in the Woods is that everything that we know from "standard" horror movies gets an explanation. The creepy warning, the reading of the Latin, the character archetypes, etc, everything is there for a reason. But one thing I just realized that I didn't understand was the two-way mirror that the Sacrifices find early in the film.
What was the point of that? It doesn't seem to play into them "choosing," like the Harbinger and the items in the basement do, and it doesn't have any apparent surveillance benefit for the technicians, given that they've got every room wired and it was covered up. I can't recall any other discoveries that show the cabin to be "more than just a cabin" like this, so it seems like it wasn't actually meant to be found. At the same time, the techinicians seem utterly unfazed by its discovery.
For a few minutes I thought it may be a hold-over from the days before the whole operation was computerized, but it seems odd for the technicians to risk entering the cabin during the ritual just to see into one of the other rooms. Aside from the risk to them personally, it would risk the Sacrifices seeing them if they ran into the adjoining room before the technician could hide.
Am I forgetting anything from the movie that revealed the purpose of the two-way mirror, or does its presence go unexplained?