I seriously doubt anyone is using poison gas.

Weaponized chemical agents work pretty fast, to the point you really won't have time to react. Specifically nerve gas. It's going to be pretty obvious from the injuries that it was a nerve agent. Dead bodies looking bad is not enough. A corpse's face rests in weird ways that look frightening. That's not indicative of anything.

Gas bombs would also be particularly stupid, when if you were going to deliver gas, you would do it carefully, like the Americans did in Vietnam. With troops on the ground. One of the reasons you really want to be sure about where your gas is going, is best exemplified by stories from WW1 like All Quiet On The Western Front. Just because the enemy is dead, it doesn't mean that the gas is inert. It could settle and stay there.

Then, of course, there's the possibility of gas actually killing people, but it not being poison gas at all. Carbon dioxide or monoxide build-up in tunnels needs to be accounted for. If the ventilation system stops, there's a good chance everyone in that tunnel inevitably suffocates.

On top of that, there are better weapons than gas for tunnel networks. Like bulldozers and thermobaric bombs. Thermobaric bombs aren't even a war-crime.

I seriously doubt anyone is using poison gas.

Weaponized chemical agents work pretty fast, to the point you really won't have time to react. Specifically nerve gas.

Gas bombs would also be particularly stupid, when if you were going to deliver gas, you would do it carefully, like the Americans did in Vietnam. With troops on the ground.

On top of that, there are better weapons than gas for tunnel networks. Like bulldozers and thermobaric bombs. Thermobaric bombs aren't even a war-crime.