This is not a phrase men normally use, so he went away looking a bit puzzled.
When he was about done shopping, a man came up to him and says, "Your fly is open."
He zipped up and finished his shopping. He then intentionally got in the line to check out where the lady was that told him about his "barracks door." He was planning to have a little fun with her.
When he reached her counter he said, "When you saw my barracks door open, did you see a soldier in there standing at attention?"
The lady thought for a moment and said, "No, I didn't. All I saw was a disabled veteran sitting on two duffel bags."