I'm sticking with the center. Looking at step 13's pic. You can see the dark piece with 2 bumps on it facing towards the camera. That is a typical IR receiver. Then to the right of that, a LED. Which lines up perfectly to that little window y'all are talking about.

So that LED must be power indicator. And the reason for the window in the casing. If made a little transparent to IR, the case doesn't need a window for IR.

KOT