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?