If we cannot directly measure or observe C it doesn't mean C doesn't exist. If we can directly measure and observe A and B it always means A and B exists. Lets say it so happens that B becomes measurable and observable only and only when A and C somehow interacts and we don't know that. Let D = A + B D is then defined, measured and therefore existent by A and B. So we don't necessarily have to directly measure or observe C but it is still needed to get D. Would we know that something is missing in definition of D? Do You consider C to be significant?