Is Silence Ambiguity or Vagueness?

In a case that's going up to the Supreme Court potentially now, there's a law that says an agency if it wishes to can require monitors on board of phishing vessels. The statute's silent. So is silence ambiguity? Is it not addressing the question? Or as some judges would say, do you simply have a presumption that something is an agency does not have a power unless the statute says they have that power? And then maybe it's not ambiguous. Since of course, statutes can never be infinitely long and can never explicitly address every possible contingency.