In my airbox there is always a little bit of engine oil that the engine spits out of the breather hose and I do'nt over fille the bike with oil....
Maybe wheely's or revving the hell out of it makes it spit out some oil
Little silver thing on the lower right of you picture is where the engine breather is coming in - gives out just a little bit of oil residue from the motor. More full you run your oil the more likely it will have a some residue. Would also come out as the moisture in the oil as it vaporizes when the bike is hot.