Could be either one, IMO.
We often say, "I
think I'm onto something," when we intend to pursue the matter further in order to become certain. Also, "I think I
may be onto something," which seems a little less certain. Still, in some contexts, we could say any of these and mean that we're certain, simply playing it coy. We may know the basic secret, but not know all the details of what it implies.
- A.