As you recall, Han lures the lone scout into a group of Rebels, and the scout surrenders immediately. That's where the armor came from.
Shortly after, as the bunker door opens, if you look in the background, you can see the scout helmet going onto someone's head. That's a rebel putting on the helmet.
There is also a picture, and I've got it on the Behind the Magic CD set, of Han tossing the helmet to the rebel, right before they enter the bunker.
I don't believe I've ever seen anything about the story behind it, so that's the best I can give you.