Those plugs are the infamous "squirrel condoms" I referred to in a post a long while ago. If you have one of those partial-throttle-on-while starting-the-bike-backfires they will blow right out of the hoses...
They are just like the rubber plugs on your intake manifold... They are inserted into the 2 breather hoses from the airbox. Squeeze the hoses and you will feel them if they are in there. You have to sort of "milk" the hoses to get them out..... Kinky, ain't it?
