It could have been a reaction to the gas, or they could've been destroyed by something else. Anyway, the bus driver seems to be a robot, especially as it's covered in blood, and the robotic "Welcome Aboard!" It is unlikely a zombie would be driving a bus with players in it while the other zombies are attacking it. Zombies may also speed up chasing the bus, because if they can run 30-50 mph chasing you on foot it's pretty much over for players. Also where are people getting the idea it's in a desert? Have pictures been released? So far it just seems like a rural town, which would have some barns nearby.