Here's a Southern account of Rucker's wounding and capture, from pages 249-50 --
http://www.archive.org/details/1861to1865byanol00dinkiala

While we were at Brentwood, General Rucker was attacked by a large force. Some of his men were driven back, leaving a gap in his line. He moved ahead with the Seventh Alabama to strike the enemy's flank, and placed them in position, then he rode a short distance to a point where he expected to find the Twelfth Tennessee, and found himself surrounded by a regiment of Federal cavalry. He at first thought they were his men, and asked for the colonel. As the commanding officer rode to meet him Rucker discovered he was a Federal, and drew his saber. The Federal officer did the same, and like game-cocks they began the battle. Rucker make a desperate rush at his antagonist, striking with all his power, and somehow dropped his saber, but instantly caught the Federal officer's arm and wrenched the saber from his hand and struck him a heavy blow with it. Very quickly he was surrounded, but with that energy, determination, and perhaps recklessness, which we had so often seen in Rucker, he tried to escape. He plunged the spurs into his horse, and forced his way, but he had gone but a short distance when the enemy began to shout, "Kill the man on the white horse," and hundreds of shots were fired at him. A man tried to cut him off, but Colonel Rucker struck him with the saber he had taken from the Federal colonel, and dismounted him. Just at that moment, however, a ball shattered his left arm above the elbow, and another killed his horse. Both fell, and the enemy rushed on him like wolves. They had no mercy for him, though he was almost unconscious from the fall. They pulled and hauled him around, swearing and calling him ugly names. He was finally moved to Nashville, where his arm was amputated. Colonel Rucker was promoted, and made a brigadier-general, but he was never able during the remainder of the war to assume command. He is now a citizen of Birmingham, Ala.

Evidently the cadet escort was elsewhere during Col. Rucker's struggle with the Federal officer. It certainly sounds as if Rucker was alone, not even a staff member beside him, expecting to find Stewart's 12th Tennessee Cavalry and not being able to locate it in the dark.

Since Rucker was alone, the flag must have been captured elsehere, too.