It's instinctive for a cat to cover up excrement and urine so, while problems in this area abound, most cats do, most always, cover up whatever they deposit into the litter box. It's not so common for a cat to cover up vomitus or expelled hairballs, but it's not unknown.
I have three cats. One of them is a 25 year-old who is not in the best of health, which might be expected at her age. Lydia doesn't always make it to the litter box, although I think she tries. Most of the messes are in the same room as the litter box, or somewhere on the way from the bed where she normally sleeps and the litter box. But even when she was younger, she wouldn't always cover her messes. She'd just walk away, and her sister would cover up for her.
My two year-old (Ella) tells me whenever Lydia has left a mess. She'll get my attention and then lead me to it, in the rare event that I haven't already smelled it, and then make digging motions next to it. She will have already tried her best to cover it up with whatever bits of cat litter might have been thrown from the litter boxes. I can always see that she had scraped everything she could find over there, trying to cover the mess that Lydia has left.
As for vomitus, my cats will sometimes vomit in the litter box, seemingly aware that it is a mess that should probably be disposed of properly, but most often they'll throw up wherever they are, and not cover it up.
