To answer this which was not directly answered in earlier answer: One of them appears to be wearing scout trooper armor without a helmet. When did this happen?? This happens when the rebels enter the bunker after capturing 'Unidentified Scout Trooper'. As they do this, behind Han, Leia etc you can see a person dressed as a Scout Trooper (by Han's leg) standing at the doorway facing outwards, adjusting his helmet as he pulls it on - presumably a rebel waiting at the doorway in case any other Imperials turn up. I have attached the closest picture to this I could find online, where you can see the rebel\trooper waiting behind the main characters, but watching the film you'll see it much more clearly including the adjustment of the helmet.
