The extra federal withholding would be included in box 2 on your W2 with the regular amount. It isn't broken out. W2 box 2 withholding goes to 1040 line 25a. Sounds like it was a good thing you had extra taken out or you would have ended up owing on your tax return. There could be several reasons like you have other income like interest, dividends or investments that don't have withholding taken out. Or you have self employment income on schedule C. Or you are married and both work.
