How absolutely engrossing. Excellent thinking, anon.
This article does a good job laying down the timing that you describe:
"[…] At one minute and 16 seconds into the speech, Trump turns around, at which point two marine sentries behind him open the doors of the west side of the capitol building to allow the group of 10 officers to walk down the steps to stand directly behind Trump.
The group, consisting of mostly junior officers along with captains and higher-ranking officers from the Army, Navy, Air Force and Marine Corps, appeared just as Trump was saying, 'Today’s ceremony, however, has very special meaning. Because today we are not merely transferring power from one Administration to another, or from one party to another—but we are transferring power from Washington, DC, and giving it back to you, the American People.'
'For too long, a small group in our nation’s capital has reaped the rewards of government while the people have borne the cost.'
At that point, the Marine officer tells the officers to disperse. […]"
https://www.wsws.org/en/articles/2017/01/23/mili-j23.html