we don't know who is attacking the hospital, maybe it is KGB, and all soldier there wear gasmask, so Mantis wearing one at that time might just be standard procedure for the mission, not to prevent other people's thought going into him. but it'd make a nice throwback for the viewer
I didn't think of that, that the soldiers wearing gasmasks there are just standard.
Hmm, I guess that would make more sense then, and that Psycho was hired with the KGB to off Big Boss. If this was true, then it might make even more sense that Volgin is the fire guy, if there are Russian ties.