What happened to my mystery snail?

So a few weeks ago I bought 2 additional mystery snails for my fish tank. One was blue and the other was white. When I added the snails to the tank the white one wouldn’t come out of its shell. And for hours it stayed at one spot. But eventually it started moving around and also started feeding. But recently it hasn’t been moving. Every morning I feed my fish and I check on my snails to see how they are doing. And I always double check my white mystery snail because I always had this fear that one day it was just going to find it dead in my tank. But I did notice that every time I feed my fish (I feed them 3 times a day, but very small portions) that my white mystery snail moved to a different spot in my tank. So I thought everything was okay again. I just finished feeding my fish and noticed the shell for my white mystery snail is empty. So my question is what the hell happened to my snail? And what could have caused this to happen?