Part left over after repair

Hi Fairphone/Ifixit,

During the repair of my Phone I was tapping it on the table to get some dust out, it came out, but also a smal plastic ring, about 3 millimeter in diameter and 2 millimeter high. It was also more narrow on one side as the other. I couldn't find out where it came from, so I put back together the Phone and it seems to be working fine. What could the (verry small) ring be? (See dropbox link for picture)

https://www.dropbox.com/s/dq4s3i57cq21ee...