One other remote possibility is that the account is a Roth account in a qualified retirement plan like a 401(k), not a Roth IRA. In that case the distribution would have codes 4 and B together, the gross amount would be included on Form 1040 line 16a and the taxable shown in box 2a would be included on line 16b. However, if this was indeed a code 4B Form 1099-R and the account had been in existence for 10 years, box 2a would have had a zero and it's unlikely that the IRS would have sent you anything indicating that you failed to report the income (since the form would be explicitly indicating that it is nontaxable).
