Something else to consider if there's not another explanation:
If the IRS thinks you have "unreported income", and you didn't, it could signal ID theft where someone could have been working illegally using your SSN, and thus their income could have been reported as yours.
In any case, if the letter doesn't fully explain the IRS's issue with your return, then you're likely going to have to talk to the IRS to see exactly what all is going on and what income they are referring to, or make an in-person appointment with them if there is an IRS office in your city. But first talk to TurboTax free Audit Support Center (or Audit Defense--if you purchased that product) as I mentioned in my first post above so they can question you about the issue, and see how they suggest you proceed.
