The $141.2 billion in Treasuries purchased into the Belgium account represents 3.2% of the total current size of the Feds balance sheet. The Fed is a private corporation and is therefore not beholden to GAAP accounting standards. However even with GAAP standards applied, a corporation does not have to itemize and disclose the details of any event that represents less than 5% of its assets. In other words, the Fed can easily bury a 3% transaction in its financial statements.
There is evidence of mismatch in Federal Reserve financial statements. For example, at year-end 2013, the size of the Feds balance sheet was $4.024 trillion.