These days, it seems like everyone in the bond market is obsessed over what will happen when the Federal Reserve starts whittling down its mammoth, crisis-era investments in U.S. government bonds.

Lost in the hullabaloo is one little-noticed fact: there's an even bigger debt pile that could draw buyers away from Treasuries at just the wrong time.

In overseas markets, more than $3 trillion of negative-yielding government bonds — which all but guarantee losses for buy-and-hold investors — have turned positive in recent months. And analysts say that number may grow over the next few years as brighter economic prospects and shifts in monetary policy lift trillions more out of sub-zero levels in Europe and Japan.