Looking for an MIA or matured savings bond might yield a cash windfall, but first, why don't we figure out what a savings bond actually is? Because in the complicated world of finance -- not to mention the maze of government bureaucracy -- the differences between savings bonds and Treasury bonds (not to mention junk bonds and private bonds) are not all easily distinguished.
First, let's talk about what a savings bond isn't. A savings bond is not marketable, meaning you can't sell it or transfer it. A savings bond is owned by the person who bought it, although you can buy it for someone else. So if you do find you have a savings bond and have no memory of buying it, no fear: You probably aren't buying government-issued securities in your sleep. Most likely, grandma bought one for you without your knowledge.
A bond is basically a loan you've given to the government, which has agreed to pay the loan back, plus interest, for thirty years. In this case, the loan is backed by the U.S. government, essentially making it guaranteed that you will earn money on the bond.
There are a few different types of savings bonds in the United States. Series I Bonds offer rates of return above inflation, while Series EE Bonds offer fixed rates of interest. They can both be redeemed after 12 months, but keep in mind that a 3-month interest penalty will occur if you do so before five years.
Remember that we're talking about government bonds here; there are also private sector bonds, which are offered by a private corporation to raise money. They usually have a higher yield but are also higher risk; they don't have the same stability as government-backed bonds. Private bonds also don't offer ownership in the company, unlike stocks.
Now that we have a firm grasp of what savings bonds are, let's figure out if you actually have a savings bond that's been effectively "returned to sender," is MIA or is just hanging around waiting for you to cash it.