Boy would command's face be red if the vents happened to be blocked and the gas goes somewhere else...like R&D department, barracks...a different cell block. You get the idea.

Luciel can't see through plant's perspective, but cutting off air to the room and sending vines to knock down vent barriers would be one hell of a distraction. Idk what kind of gas it is, though, probably the deadly kind.
I mean, heh, as long as we're metagaming about knowing the girl is a guy from almost half a mile away, I may as well know about the gas.
To reason it: Luciel had broken out, before. The gas trick seems like a thing that would have been tried.