I actually saw one despawn in front of me. It was one of those bloated 4 legged creatures. I got ticked off at it walking all over my base and decided that I just wanted to see it dead, so I was chassing it around with my zeno zapper. It ran near one of those poison nests and just vanished out of thin air. No body, no shell, nothing at left at all of it's existence. Given the location where that happened, it might have fallen under the mesh. I've seen plenty of monster walk through poison without any consequence, so I don't have any reason to beleive that was it died to poison.
There was also a case where I saved in close proximity to one of those fire spitter, and reloaded the game after a bad fight. After I reloaded the game, I could hear it but could not see it. I eventually found it behind the mesh by peaking around with the camera.