I'd probably just take all the fairings off and check every possible cap just to be on the safe side. Assuming you haven't already done that.
But, it could just be some random cap that had fallen off some other vehicle and gotten somehow stuck or wedged under your fender and just happened to come off when you were at home. I'm sure weirder things have happened and if you double check all the caps that you should have, the only other reasonable explanation would be that somebody dropped it somewhere in the chassis and it finally worked its way free.
Going to the lock focus site reveals that they sell caps like that for many different vehicles and even office furniture. So, it might not even be from a vehicle.
Oh, and it's not necessarily a vent hole. Caps somewhat similar to that sometimes are meant to have a bolt that goes through that center hole. IIRC, the oil filter is held in place by a cap with a hole like that. But, it's pretty clear that this isn't from that. I doubt you'd notice the complete lack of oil in your engine or the huge amount of oil on the floor.
Last edited by pfranks; 07-09-2016 at 04:18 PM.