[The refusal is raw across his nerves, subconsciously against the grain of what he feels he is owed, and what he feels is most reasonable in this situation. To know about others who are comparable enough to shinobi that he can sense them — it's certainly the priority. Why wouldn't Itachi wish to share that information with him?]

If you're refusing to tell me who they are, then you have a reason. Either they aren't a threat or you don't want me to seek them out because they are.

[Not knowing his brother's mind, he wonders if these people are dangerous. If that's the case, he'll prove himself, because it would be worse to think that Itachi believes he's unable to handle that risk.]

I can always go find them right now.