Since it's that large and also black, it is in my view most likely to have come off the plastic side covers that cover the cables and other things behind the triple tree. Between the tank and the front forks.
The radiator shroud has mostly very similar/identical bolts but all the ones that length are shiny/chrome. Some of the others are shorter but still shiny (if you have a chrome shroud). The black ones on the radiator are inside the shroud holding the actual radiator grill in place, and they're smaller and shorter and not externally visible.
Btw, check all the bolts if you haven't. Without loctite in my experience a number of the radiator shroud bolts vibrate loose almost instantly, I put blue loctite on all of them when I mounted my radiator grill embellisher from Flip and have had no worries about that since.
(All this written from the viewpoint of an R3 Touring owner with a chrome shroud, if your bike differs then perhaps it uses long black bolts like that on the shroud).