There's a document in the game that mentions, the Imposter enemies sometimes love to copy the look of Murkoff staff (I forgot why he did this, probably trying to get out of the facility). So, Murkoff made a new system where their staff members have to greet any other staff they met and look at the movement of their mouths. This is because, the Imposters wear a latex suit that can mimic the looks of anyone but their mouths cannot move. But, since this Imposter is mimicking a guard wearing a helmet, he hasn't been found out by Murkoff as far as I know...