I've read he may've went downstairs to watch the rest of the play or went for a drink.
Parker was fired from the police force in 1868 for sleeping on duty. He went back to being a carpenter.
He may have taken a break
His bodyguard Willian Crook had the night off. The rumors say that he was drunk in a bar with John Wilkes Booth.
On that fateful night in 1865 at Ford's Theatre, Abraham Lincoln's sole bodyguard, Washington policeman John Frederick Parker, was supposed to be sitting outside the presidential box in a passageway behind the door. Trouble was, Parker couldn't see the stage from there, so he left his post to get a better view. This was after he was three hours late to relieve the previous bodyguard earlier in the afternoon. Even worse, during the intermission Parker went out for drinks with the coachman and footman of Lincoln's carriage. He was not at his post when John Wilkes Booth entered the president's box.
Off his post
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