barracks door

Mr. Johnson got himself a new secretary. She was young, sweet and very polite. One day, while taking dictation, she noticed his fly was open.
When leaving the room, she said, "Mr. Johnson, your barracks door is open."
He did not understand her remark but later on, he happened to look down and saw that his zipper was open. He decided to have some fun with his secretary.
Calling her in, he asked, "By the way, Miss Jones, when you saw my barracks door open this morning, did you also notice a soldier standing at attention?"
The secretary, who was quite witty replied, "Why no sir, all I saw was a little disabled veteran sitting on two duffel bags."