Original post by: Sam Goldheart ,
Hey Alexander! That looks like a gasket that belongs on the front panel sensor assembly (the front-facing camera, ambient light sensor, and microphone cable you pulled off the top of the phone). This particular rubber bit belongs on top of the ambient light sensor. We don't have a great image for you, but I found this unsourced one in a broken link on Google images. [image|1365945] Your phone will likely be fine without it, but it may struggle in dimming when you make calls and other ambient-light issues, since this helps shade that sensor.