I was viewing a youtube video that was about thermal imaging and saw something that caused me to ponder but I couldn't figure out an answer and the one assumption I have I don't know if it's correct. The video was of a man and while he was doing an experiment I noticed his reflection in the glass door behind him about five or more feet away. How is that possible? The thermal camera should only pick up heat released from objects but the door shouldn't release a perfect reflection (not blurred or fuzzy -- looked just like him just a little dimmer). I thought maybe the heat from his body is bouncing off the glass but that doesn't sound right. A moment later I noticed it was picking up his cat's reflection as well and it was father from the door than he was.

I'm not going to pretend that I understand all the ins and outs of thermal imaging, I don't, so can someone please explain to me how this is possible?