If you are missing some U.S. savings bonds, it is unlikely whoever made off with them would be able to cash them. A savings bond can only be redeemed by a listed owner, and the redemption rules make it difficult for someone to pretend to be you. You will want to apply to the U.S. Treasury as soon as possible to have your stolen bonds replaced.
TL;DR (Too Long; Didn't Read)
While it isn't entirely impossible for someone to cash a stolen savings bond, it is highly unlikely. The individual would need to prove he was the listed owner by presenting a valid form of identification.
Cashing at Bank
You can cash your savings bonds in person at a bank. If they are worth more than $1,000, you must have an account open there longer than six months or have your identify verified by another customer who knows you. Less than $1,000 in bonds can be exchanged for cash at any bank with valid picture identification. Acceptable IDs include a driver's license, passport, employer ID with a photo or trade license with a picture. The bank officer cashing a bond must make a record of the name, address and identification information of the person cashing the bond. It would be tough for a thief to get past all of these tests.
Redeeming by Mail
You can redeem savings bonds by mail, sending them to the Treasury and getting a check in return. However, for this option, the bonds must first be taken to a bank to have the owner's information verified by a bank officer. The address a check will be mailed to must match the address on the owner's ID. It is unlikely a thief would try to cash stolen bonds by mail and then try to cash a government check issued in your name.
Replacing Stolen Bonds
If your savings bonds have been stolen, you can send a request to the Treasury to issue replacements. Use Treasury FS Form 1048, "Claim for Lost, Stolen or Destroyed United States Savings Bonds," which you can download from the TreasuryDirect website. The form includes directions and the address where it needs to be mailed. You must go to the bank to have your signature verified by an officer there before mailing the form.
Although it's a little late to take precautions once your savings bonds have been stolen, you may want to spread the word to help others protect theirs. Keeping your bonds in a safe deposit box provides much better security than that drawer in your desk. A further safeguard is to keep a separate list of your savings bond serial numbers, which will make it easier to request replacements. Another option is to convert your bonds to electronic form and hold them in an online account on the TreasuryDirect website.