Looks like the rubber piece that goes between the mainboard and the front glass to seal the proximity and brightness sensor on the mainboard. Here it is in a teardown video I found.
[image|3315371]
And the corresponding sensors on the main board, circled in red:
[image|3315370]
I really don't think it's worth re-opening the phone to place it back in, unless you notice any issues with proximity sensor which I believe is used to know when the phone is up against your ear taking calls to black out the screen, or the brightness sensor used to adjust screen brightness automatically. But I doubt there will be any issues without it