So during the final episode, the group made it to yangdong and if you look closely to the zombies. They weren't shown to have bite marks or wearing any of the hyosan occupations uniforms, my theory is that sun-hwa after being bitten escaped from school and she made it to yangdong where she could possibly be eating the residents to stay alive. Sun-hwa wasn't even shown in the bombing and as also Yangdong weren't shown to have any technologies, it would've been likely that yangdong doesnt know about the evacuation