Before I went to sleep, I went to turn off the tank light, and wondered why I only saw one fish, one large snail, and three small purple snails, when there should be four of them. I counted. I counted again. I looked under things.
Then I remembered pushing one of them away a couple of weeks ago when he'd tried climbing up the impeller intake, which leads straight up to the aforementioned filter casing. Took off the top and looked in. Couldn't see anything. Went to get a flashlight. Ah.
Brief hopes that it wasn't too late to rescue the snail trapped in the filter casing evaporated when I realized there was only a shell left. The impeller mechanism is probably not designed with small, curious intruders in mind.
I guess this one was the smallest of them; maybe the others are too big to fit. I'll also contemplate ways of blocking the openings for the future.