Fed May Buy $300 Billion in Treasuries After QE2
Not that stealthy or unannounced. They already did announce the reinvestment of proceeds, and also stated that they want to keep the balance sheet at the current amount (which would require reinvestment of most proceeds from maturing instruments; otherwise it would shrink as instruments mature).
What is sort of stealthy (and strangely wise, IMO) is the movement in type of instrument from maturing mortgage and other paper to Treasuries. If you believe in the safety of USTs (lol), this will make the balance sheet look stronger.