I have a Vespa Grande. I was just adding some oil to the rear hub (thanks matt and zippy for all your help with that) and saw a little peice under the hub nut. It has a plastic black cap on it and it can be pushed in and if you pull down the hinge it releases. what is this?? do i need to do something with it? On the side cover it has a hole for it... good enough description? I can post a picture if needed.