What the heck is this?!

Yesterday when i closing one of the CPU case and when i turn over, something drop out. Normally i will think a screw drop off or jumper, but this....i don't really know. I checked the whole MOBO again, but i found nothing been broke. Headache ler...immediately i try to switch on the PC and see if it function properly, nothing wrong...Hmm....what the heck is that?