Are you sure? Because I read it as "some of them (the decoys) were inside and others weren't." I also added that they were using decoy slaves because without them the decoy wouldn't work. Otherwise that would be pretty stupid of Mois and his men to just bottleneck like that. The rebels would say hidden as well, because why go out into the open when you can fight from the shadows? 

Basically, I thought that the rebels were shooting the decoys through the walls, doorways, and windows of the warehouse, which is just one gigantic building. Meanwhile, Mois and his other men were picking them down one by one. And Erik could be captured just as they were about to storm the place and take the "survivors."