Interestingly (or perhaps not) unclaimed federal tax refunds are not reported and remitted as unclaimed property, because under federal law:
No overpayment of any tax imposed by [the Internal Revenue Code] shall be refunded (and no interest with respect to such overpayment shall be paid) if the amount of such refund (or interest) would escheat to a State or would otherwise become the property of a State under any law relating to the disposition of unclaimed or abandoned property.
Source: 26 U.S.C. 6408.
Accordingly, taxpayers who believe that they are missing refunds should contact the IRS via the "Where's My Refund" tool on the IRS website.