Yeah, looking into the radiator would be better than checking the reservoir, if there is oil in the coolant, it might not be circulating into the reservoir. But it would collect as a film on the top of the coolant in the radiator tank itself, and you should see any if it is present.
Also it's possible that water could leak into the oil but not vice-versa. If it's a leak where the water pressure would be greater than the oil pressure. A common place for leakage is the O-rings in the head gasket, or the gasket itself wasn't installed correctly at the factory.
And it's still very possible that it is just condensation.
One way to tell is to do a pressure test on your cooling system. Such a small leak may be hard to detect. You'd have to pump it up and leave it for awhile to see any drop in pressure.
Sky, you have to remember i'm just guessing here... I've never been inside the new B1250 motor, but I have encountered this problem in other water-cooled motorcycle engines, and a crapload of car engines. In my experience, with such a low-mile engine, it usually turns out to be an incorrectly installed O-ring or head gasket. Rarely is it a cracked head or cylinder casting in a fresh motor like yours.
And I keep thinking about the condensation thing too.