If it's holes being bored then I'm thinking "other gastropod" (eg. oyster drill or similar).
Yeah, dead snails are one of the foulest smells in the hobby. Dead anemone is pretty bad too. I think snails (ie. when there's still a bit of flesh left to rot) are probably the worst smell I've encountered so far. And a hobby like ours with so many fragrant aroma's .. that's saying a lot.
