If you have capital gains only from Forms 1099-DIV, no Schedule D will be included in your tax return. Instead, the "not required" box on Form 1040 line 7 is marked. With a mailed tax return, it's not uncommon for the data transcriber to overlook the marking of this box, particularly in this time of the IRS having less experienced and overworked data transcribers. You can determine if this has happened by obtaining your tax return transcript for the year in question and comparing the transcript to what you filed.
Usually when there is an amount on Form 1040 line 7 and the transcribed tax return has no Schedule D or marking of the "not required" box the IRS will ask you to supply what it thinks is a missing Schedule D. It's usually easier to just comply with that request instead of trying to explain that no Schedule D was required and that the IRS erred in not transcribing the "not required" box marking properly.
