Pet snail, previously okay, found missing its body?

[comesnailaway]

The other day, while checking on my pet snails (as I do daily), I discovered that one of my snails was missing its entire body?
The only trace of the snail that was left was its shell, which upon inspection was seemingly completely abandoned, almost as if the snail had just crawled out like a hermit crab.
I can't find the body anywhere and doubt that it was eaten by another snail, since all the snails I have are herbivores as far as I can tell (I have all garden snails except for one snail which I found outdoors, upon research seems to be neohelix abolabris).

Any ideas on what could've happened? Fruit flies do infiltrate my enclosure for my snails, could they have possibly eaten the snail?
What can I do to prevent this from happening again?

[Liguus]

When snails die they usually retract deep and they will decompose relatively quickly. Especially with a small snail I would not be surprised to find a mostly empty shell after only a few days. With fruit flies/maggots that process will be quicker.

I can't say anything for sure without seeing a photo of the other snail, maybe the Neohelix is a carnivorous Haplotrema? Obviously it's just guessing at this point, but sometimes even a Neohelix will scavenge and could have eaten the snail after it died.