Well least so far I found many flags that people never thought were captured.
1) Custer's never lost a flag. Custer's Headquarters Swallowtail flag was recaptured at Saylor's Creek and is currently at the MOC.
2) The 12th Corps "Never lost a flag or gun." Well the 46th PA at Chancellorsville broke that.
3) The 2nd Corps "Never lost a flag before Spotsylvania." The Irish Brigade lost several colors at Fredericksburg ending that rumor. At Chancellorsville the 145th PA lost a flag, along with several officers and 150 enlisted men. The Confederates reported capturing a flag with "145" on it. 145th PA was in the 2nd Corps. At Brandy Station the 66th New York lost a guidon. At the Wilderness when Longstreet's Corps hit the 2nd Corps, Mahone reported that his brigade alone, captured "several stands of colors." Only the 2nd Corps was there.
I believe the 2nd Corps lost flags in just about every major battle. Antietam (Sedgwick's Div) (59th NY, 42nd NY and the 7th Michigan) I have my eyes on. Gettysburg, at least one flag from the 2nd Corps had to have been lost, but never reported. I'm not talking about Pickett's Charge but July 2nd.
Well I glad many others see the trouble of finding Union captured flags. I think I have found as many captured Union flags as I am going to find "popular" ones. OR's, Returned War Department flags, books, magazines, etc.
But I am not or never going to give up. I understand why nobody has done this before.
Have a good morning, keep up the good work!!