What is this part?

I've torn down several PS4 Slims for the purpose of cleaning and testing. I'm currently working on my first CUH-2215, trying to take careful note of the differences I see. I'm at the point seen in the first picture.

I was trying to take note of the thermal pad locations since they sometimes stick to the metal plate instead of the chip. It looked to me like one was unaccounted for (I was wrong), so I flipped it upside down to see if what I thought was an errant pad would reveal itself.

Instead, the little part in the second picture fell out. It looks like a small wingnut. I'm not sure what it is or where it goes. Do any of you know? I'm assuming that when I go to put it back together, I'll find that there's a screw with nothing to screw into and I'll have my answer. Hoping to save some time. Thanks!