I have a magnifying mirror attached to the bathroom wall cupboard on the opposite side of the bathroom to the window and sometimes when I am standing in front of the mirror and the sun is shining through the window, it reflects off another mirror and burns me in the back. When it first happened I wondered what it was. Now I can move out of the way. There is nothing to catch fire there, but it would be catastrophic if I didn't realise what it was and my shirt spontaneously combusted while I was wearing it.
My point is that the mirror does not need to be directly inside the window - it can reflect the sunlight at any point in the room.