Poison gas almost always comes from a hole in the ground. You have to discover it with perception (which usually requires you to be in the poison).
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You might want to throw stuff on the hole anyway so the poison does not return when the spider moves, but sending the pet in the poison was a good way to find the hole without damaging your chars.
I had a problem with a cask breaking when I threw it at the vent one time, so I tend to take the walk in and drop approach. To be fair, that was a throw from inventory, rather than a move from the ground next to me. I couldn't drop anything else on the vent because of the debris