It may very well be that the plastic piece broke off. At the very least I would change the oil immediately and look for plastic shavings. If the bike is under warranty, I would have the dealer pick the bike up, pull it apart and throughly check the bike out before you started it up again.

 

Again there is no sign of anything broken off. All I see on the inside of the cap is the cross from the casting mold. Nothing that could possibly hold a plastic dipstick.

 

Are you sure your not looking at the radiator cap????

 

dont worry its NORMAL