When Benjamin saw the gas he went into an almost automatic state of alarm, though managed to restrain himself from reckless action due to a combination of risk of getting shot and the knowledge that the police department were above gassing citizens. If they didn't care about the citizens inside they would have just blown the bus into the after life after all. He covered his face and weathered the panic. 

According to the Lily, who was now examining the fallen criminals, he was right, they had been zombies. However there was something not quite right. He felt like all his hair was standing on end. He had fallen out of his equilibrium. It was then that he noticed that while the glowing marks had vanished from the mens' necks (not surprising if they were 'reanimated'), they had been almost transferred to the necks of the passengers including, even more worryingly, his own. 

'Excuse me coroner, do you have any idea what these marks could be?' He asked in a quiet, tone, with only the faintest note of worry tinging his voice. 

If she didn't know he'd take it to the voodoo queen, in Pyrrhic city problems are scientific or magical, luckily he was now in contact with an expert in both. 

'Also, should I see an doctor about exposure to that gas?'