…whoever, in any matter within the jurisdiction of the executive, legislative, or judicial branch of the Government of the United States, knowingly and willfully
(1) falsifies, conceals, or covers up by any trick, scheme, or device a material fact;
(2) makes any materially false, fictitious, or fraudulent statement or representation; or
(3) makes or uses any false writing or document knowing the same to contain any materially false, fictitious, or fraudulent statement or entry;
shall be fined under this title or imprisoned not more than 5 years, or both.
18 U.S.C. § 1001(a).
Martha's problem was violation of (a)(2), in that the jury found that she made a false statement or representation (and probably implicated (a)(3) as well with the phony logs). However, remaining completely silent in the face of otherwise reasonable questions may, in the hands of an enthusiastic prosecutor, cross the line on (a)(1). Certainly not in all circumstances; but consider this possibility:
Government Investigator X is attempting to determine whether Megacorporation M had engaged in fraudulent accounting practices. X asks administrative assistant A whether she knows of any documents that might tend to prove or disprove such practices. Although A does not know the contents of the documents and is not privy to the actual details (we'll assume for the moment that M was guilty), she does know that her supervisor, Vice President V, has directed a massive shredding operation that is to begin within the next few minutes. Fearful for her job, but not wanting to lie, A refuses to answer the question and demands that X obtain a subpoena before she will answer. While X is obtaining the subpoena, V's shredding operation eliminates all of the admissible evidence of M's wrongdoing.
Although it's hardly fair, this appears to violate the "conceals… by any trick" the material fact of the shredding under circumstances in which the shredding itself results in the destruction of evidence.
What this really points out is that § 1001 is badly drafted. Compare it to the clarity of Article 107 of the UCMJ (10 U.S.C. § 907):
Any person subject to this chapter who, with intent to deceive, signs any false record, return, regulation, order, or other official document, knowing it to be false, or makes any other false official statement knowing it to be false, shall be punished as a court-martial may direct.
Typical. In the military, we're told to ensure that orders (and directives) can be understood by the people who might actually have to follow them. Apparently, the wisdom of that approach has not reached Capitol Hill.