You would see little black bugs crawling on him.
They usually like to hang out around the eyes of an animal or dig in under the arms and legs in the folds of the skin. They usually have a few shedding problems too because of them.Or you'ld find some drowned in your leo's water bowl maybe.
I would think it was an impaction. Impacted geckos drink alot too. Just because you didn't see him eat the sand doesn't mean he didn't.Unless you kept a good eye on him all night and day. And if you've been monitoring him closely enough you would have seen the mites. An infestation bad enough to kill would have been noticable. In an older reptile manual it even says that mites are annoying, but rarely fatal.
I've never seen a leo with mites, but I'm sure it happens occasionally, but they're not the type on animal they like to frequent on. Usually you see them on animals with larger/thicker scales. Iguanas, pythons, Collared lizards... animals that they can find large overlapped scales to dig under.