Where did my CoreStorage partition go? I have a 2015 MacBook Air 128GB with Sierra Recent purchase as new from Apple, still under warranty). A couple of months ago I was looking at the SSD using DiskUtility and saw that the SSD seemed to be a lot more than 128GB, it looked like it was 256GB. 128GB was available to me and the rest was a CoreStorage partition. (Give or take a few GB for the Recovery partition.) So I thought that if I could remove the CS partition, I would have 256GB available to me, not 128GB.

A couple of manoths later, I finally got time to try it. Much to my surprise, there is no CS partition anymore. HD Utility doesn't show one, so I tried Terminal and still no CS partition, whereas previously there was a CoreStorage partition shown. And the SSD is only 128GB.

So where did the CS partition go?
It is possible that you misinterpreted what you saw. The diskutil list command in Terminal, for example, would show you both the 128GB physical disk, as well as the 128GB CoreStorage logical volume. These are listed separately, but are in fact the same storage.

Running disktuil cs list in Terminal will confirm if you have a CoreStorage volume or not. To my knowledge, it's not likely that a CoreStorage volume would revert itself. One possible cause that I can think of would be a reinstall of macOS.