i just removed my front sprocket cover and this small black rubber cap fell out. I have no clue where it came from. It looks like a valve stem cover from a tire without the inside threads and is made of rubber.
http://www.rc51forums.com/forums/attachment.php?attachmentid=93314&stc=1&d=1441647133
http://www.rc51forums.com/forums/attachment.php?attachmentid=93314&stc=1&d=1441647133