They would at least notice Tuuri is gone (Tuuri who has no business being outdoors unsupervised, let alone wander at a big distance from the camp alone) and know something Bad has happened.
I was going to give all the reasons they'd figure out what happened, when I remembered my theory about miscommunication being the underlying premise of the comic.
They're certainly going to know Tuuri's gone. Will they follow Lalli, who just ran past them screaming for help in a language none of them understand? Will they notice the extra set of prints that he's following, after they all run over the top of them, and realize that Tuuri went into the water on her own feet, rather than having been dragged in by something? If they get that far, I think they'd have to figure out that she must have become symptomatic, even without the extra clue of the conversation she had with Lalli.
But I find myself with the unpleasant thought that they might shrug and turn back to dinner, thinking 'who knows why that scout does what he does?', and not figure out that Tuuri's missing until later; at which point I suppose they might think she was dragged off by a troll that had made it close to the tank, especially if the prints are gone by then.
-- I agree that Lalli will stick it out. He didn't want to come in the first place, and has expected disaster for a long time now, but he's kept doggedly (even though he's a cat) doing the job he's asked to do. I expect he'll keep on doing it.
